This onion tart is deeply savory with caramelized onions, mushrooms and mustard, and is super easy 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.
Updated April 14, 2022
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.
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.
Ingredients for Onion and Mushroom Tart
- Frozen Puff Pastry - I like DuFour's Puff Pastry best, but I also use Pepperidge Farm Puff Pastry with great success.
- Caramelized 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. Do not use red onions.
- Caramelized mushrooms - simple white button mushrooms work well in this recipe.
- Mustard - any brown or grain mustard is ideal. I use a Dijon mustard. 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.
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Step 1 (15 minutes prep + thaw and bake time)
Roll and shape the puff pastry.
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 1 inch off of each side. Baste the edges of the square pastry with water using either a basting brush or your finger. Then place all the cut edges onto the pastry square, forming a frame. This raised area will be the crust for the tart.
Bake the puff pastry according to the package directions..
Step 2 (60 minutes)
Make the tart filling.
Thinly slice the onions and sauté in a pan over medium-low heat with some butter for 45 minutes. Chop the mushrooms and sauté In a separate pan with a knob of butter, and fresh thyme, over medium-low heat until the pan is dry. The mushrooms will be browned. Remove the thyme twigs.
This step can be done up to a week ahead.
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)
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.
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.
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5 Ingredient Onion Tart with Mushrooms Recipe
- 1 frozen puff pastry
- 2 teaspoons Dijon mustard
- 2 ½ pounds yellow onions, about 6 cups when thinly sliced
- ¾ teaspoon kosher salt, divided (or half this amount with table salt)
- 1 cup finely chopped white mushrooms
- 3 tablespoons unsalted butter, divided
- 8 twigs fresh thyme, divided
- 9 roasted cherry tomato halves or sun-dried tomatoes (optional)
- 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 1 inch off of each side. Baste the edges of the square pastry with water using either a basting brush or your finger.
- Then place all the cut edges onto the pastry square, forming a frame. This raised area will be the crust for the tart.
- Line a baking sheet with parchment paper or foil, and place the puff pastry on it. Using a fork, prick the bottom of the tart numerous times. This will allow steam to escape without billowing the tart bottom up. 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.
- When cooled, spread the bottom of the tart with the Dijon mustard. You don't need much - it just adds depth of savory-ness.
- Thinly slice the onions using a sharp chef's knife. Add 2 tablespoons butter to a skillet and melt over medium-high heat. Pile in the sliced onions with ½ teaspoon salt, using tongs to toss and coat with the butter. The onions will begin to relax in 5 - 10 minutes. At this point, reduce the heat to medium. Sauté for 45 minutes, or browned.
- Finely chop the mushrooms and sauté In a separate pan with 1 tablespoon of butter, 5 twigs of fresh thyme, and the remaining ¼ teaspoon of salt over medium-low heat. Occasionally move around the pan, and sauté until the pan is dry. Mushroom contain a lot of liquid, and this can take 20 minutes or more. The mushrooms will be browned. Remove the thyme twigs when finished.
- Add the mushrooms to the onions and stir to combine.
- Lower the oven temperature to 350˚F. Fill the puff pastry with the onion filling. Add the tomatoes if using. Bake for 10 minutes. Remove the tart from the oven and lightly sprinkle fresh thyme on top before serving.
- Serve warm. Slice in half or quarters. For an appetizer, slice into small squares.