This onion tart is deeply savory with a ton of umani with caramelized onions, mushrooms, mustard, and roasted tomatoes. It's super easy to make with frozen puff pastry.
Cut this tart into squares for a hand-held appetizer, or serve with a salad for a light dinner. Make the filling ahead, if you're pressed for time, and bring back to room temperature before filling the pastry and baking.
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This French Onion Tart with Mushrooms is perfect for chilly nights with its slightly caramelized onions and mushrooms. For parties, I serve it as an appetizer, pre-cut into individual squares that can be easily hand held.
But it's also great for a light dinner with a salad.
The best onions to use in this recipe are yellow onions. And if you've ever wondering the best way to store them in your kitchen, read this article on whether onions should be refrigerated. I admit, my research completely changed the way I store them.
Frozen puff pastry is easy to work with, whether you're a beginner or expert in the kitchen. For the less experienced cook, detailed steps with photos are provided below, along with useful tips for making the perfect French Onion Tart.
🧅 Ingredient Tips
- Frozen Puff Pastry - I like DuFour's Puff Pastry best, but I also use Pepperidge Farm Puff Pastry with great success.
- Onions - Yellow onions are my first choice for this recipe, but sweet onions will work fine. Examples of sweet onions are Vidalia and Walla Walla. And if you've ever wondering the best way to store them in your kitchen, read this article on whether onions should be refrigerated. Do not use red onions.
- Mushrooms - Simple white button mushrooms work well in this recipe but if you want a more complex flavor use two different kinds of mushrooms.
- Mustard - Dijon brown or grain mustard is ideal. Do not use yellow mustard.
- Tomatoes (optional) - either grape or cherry tomatoes are perfect for adding as a topping. I make balsamic roasted tomatoes a bunch at a time, and keep them in a jar in the refrigerator for adding to dishes at the last minute.
📝 Instructions Overview
Detailed instructions for making this French Onion Tart are in the recipe card below, but here's an overview!
Step 1
Roll and shape the puff pastry. Thaw the puff pastry according to the package directions and lightly roll it out using a rolling pin. Slice a strip from each side and place them onto the pastry square to form a raised frame.
Bake the puff pastry according to the package directions. When lightly browned, cool and turn down the oven temp.
Step 2
Sauté. Sauté the onions and mushrooms in separate skillets, and then combine them with the thyme.
Step 3
Fill the tart. Spread the bottom of the tart first with mustard and then the onion and mushroom filling. Top with roasted tomatoes (if using).
Step 4
Serve. Sprinkle with the remaining fresh thyme and serve warm.
This step can be done up to a week ahead.
Pro-tips:
When slicing the onions, first cut them in half, peel the outer layer away, and slice the onion halves along their lengths. This allows them to lay flatter in the pan.
Purchase pre-sliced mushrooms, stack a few on top of each other, and slice into slivers.
Once the onions soften, don't nudge them around the pan very much. Leaving them in place allows them to brown better. Sauté the mushrooms the same way.
Step 3 (optional)
Slow roast cherry tomatoes.
Roast the tomatoes while the onions and mushrooms are cooking. First, cut cherry tomatoes in half, drizzle with oil and balsamic vinegar, and roast for 45 minutes at 350˚F. They can be made up to 2 weeks ahead and saved in the refrigerator.
For more details, go to this recipe for Balsamic Roasted Tomatoes. They're honestly one of the best condiments to have in your kitchen. And the recipe is the #1 hit on google for balsamic roasted tomatoes!!
Step 4 (15 minutes)
Final bake.
Thinly spread Dijon mustard on the base of the pre-baked puff pastry, and then fill it with the tart filling. Top with the roasted tomatoes, if using. Bake for an additional 10 minutes. Remove from the oven and sprinkle with fresh thyme.
Step 5
Serve.
To serve, slice it into the size squares you want.
Cook time note:
While the frozen puff pastry thaws and bakes, prep the onions, mushrooms and tomatoes, if using them. Sauté the onions and mushrooms while the tomatoes roast. The onions, mushrooms and tomatoes can be started while the puff pastry bakes.
For another savory tart, check out this roasted cauliflower tart with kale, caramelized onions, tomatoes. It's all baked in a cheddar cheese crust.
5 Ingredient Onion Tart with Mushrooms Recipe
Print Recipe Save Pin Recipe Add to Recipe Collection Add to Shopping ListEquipment
- rolling pin
Ingredients
- 1 frozen puff pastry
- 9 roasted cherry tomato halves or sun-dried tomatoes (optional)
- 2 ½ pounds yellow onions, about 6 cups when thinly sliced
- 3 tablespoons unsalted butter, divided
- ¾ teaspoon kosher salt, divided (or half this amount with table salt)
- 1 cup finely chopped white mushrooms
- 8 twigs fresh thyme, divided
- 2 teaspoons Dijon mustard
Instructions
- Preheat the oven to 400˚F. Thaw the puff pastry according to the package directions, and carefully unfold it. The dough will look a little rough on the folds, so lightly roll it out using a rolling pin to even out the surface.
- Use a sharp knife to slice a one-inch strip off each side. Baste the edges of the larger puffed pastry rectangle with water using either a silicone basting brush or your finger. Then place the strips onto the pastry rectangle to form a raised frame. This raised area will be the crust for the tart.
- Line a baking sheet with parchment paper and place the puff pastry on it. Using a fork, prick the bottom of the tart numerous times. This allows steam to escape without billowing the tart bottom up while it bakes. Bake until lightly browned, about 25 - 30 minutes. If the tart bottom billows, prick with a sharp knife and gently press down. There's no need to completely flatten it, since the filling will weight it down.Allow to cool and roast the tomato halves (optional) if you plan to use them following the linked directions.
- While the puff pastry bakes, thinly slice the onions using a sharp chef's knife. Add 2 tablespoons of the butter to a large skillet and melt over medium heat. Pile in the sliced onions with ½ teaspoon salt and use tongs to toss and coat with the butter. The onions will begin to relax in 5 - 10 minutes. Sauté for 45 minutes, or browned. When the onions begin to brown, reduce the heat to medium-low and don't move them around often so they caramelize.
- While the onions caramelize finely chop the mushrooms and sauté In a separate medium-size pan over medium heat with 1 tablespoon of butter and half of the fresh thyme until well-browned.Once the mushrooms are caramelized, remove the thyme, and add the mushrooms to the onions along with the remaining salt.Pro tip: Only occasionally move the mushrooms around the pan as they sauté.
- When the tart cools, spread the bottom of the tart with the Dijon mustard. You don't need much - it just adds depth of savory-ness.
- Lower the oven temperature to 350˚F. Fill the puff pastry with the onion and mushroom filling. Add the tomatoes if using. Bake for 10 minutes. Remove the tart from the oven and lightly sprinkle the remaining fresh thyme on top before serving.
- Serve warm.
Liz
Oh, yeah! I'd definitely make this my dinner! Love that you added ttomatoes and mushrooms---even more enticing!!
The Wimpy Vegetarian
Thanks so much Liz! It was so so good!
Dorothy at Shockingly Delicious
You are queen of the elegant tarts! Thanks for sharing your tip on how to get the raised edge...so helpful.
The Wimpy Vegetarian
Awwww, thank you, my friend!
Jane
What a stunning side dish Susan - one that could easily be a vegetarian main course. Absolutely beautiful!
The Wimpy Vegetarian
This is totally a main course for me!! And Carnivorous Maximus, my husband, kept wanting more!
Karen
This is so elegant and sounds amazing! I love your outer "crust" too!
The Wimpy Vegetarian
Thanks so much Karen!! I like an outer crust, but it's totally not necessary 🙂
Jo
Where did you use Dijon mustard ?
The Wimpy Vegetarian
Thank you so much for asking this question. I mistakenly left it out of the instructions. I've now inserted it, and in the correct place. Once the tart shell has cooled from baking, spread the mustard on the bottom of the tart. It adds another layer of deep savory flavor, but can be omitted if you prefer not to use it. It doesn't take much, but I could tell the difference when I added it. Thanks so much again for letting me know it was missing in the instructions!