This French Onion Tart with Mushrooms is perfect for fall with its slightly caramelized onions and mushrooms. Normally, if I was hosting parties right now, I’d serve it as an appetizer, pre-cut up into individual slices.
But I’m not having many gatherings right now, and I’m not sure when I will.
So, it’s dinner with a salad. And it’s perfect for anyone to make, whether you’re a beginner or expert in the kitchen. For the less experienced cook, detailed steps with photos are provided below and in the the recipe box. Additionally, I provide tips for making the perfect French Onion Tart.
French Onion Tart Components
I like DuFour’s Puff Pastry best, but I also use Pepperidge Farm Puff Pastry with great success.
Yellow onions are my first choice for this recipe, but sweet onions will work fine. Examples of sweet onions are Vidalia and Walla Walla.
Simple white button mushrooms work well in this recipe.
Slow Roasted Cherry Tomatoes (Optional)
The roasted tomatoes are optional, but they add a burst of brightness. Either cherry or grape tomatoes are perfect here. If you wish to simplify this recipe, skip the tomatoes.
Steps in Making a French Onion Tart
Roll + 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.
Sauté 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.
Combine the the onions and mushrooms. This step can be made up to a week ahead.
Pro-tip: When slicing the onions, it’s best to 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.
Pro-tip: I purchase mushrooms that are pre-sliced at the market, stack a few on top of each other, and chop into the size I want.
Pro-tip: 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.
Slow Roasted Cherry Tomatoes (Optional)
If you decide to add roasted tomatoes, roast them up while the onions and mushrooms are cooking. Halve cherry tomatoes, drizzle with oil and balsamic vinegar, and roast for 1 hour at 350˚F. I have roasted tomatoes around my kitchen ALL the time and add them to oodles of dishes for an added burst of umami flavor. They can be made up to 2 weeks ahead and saved in the refrigerator.
Here’s a post on slow roasting any tomato all year long. 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!!
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 in half or quarters. I have this with a kale salad tossed with apples and dressed in a simple vinaigrette.
This post may include affiliate links, which help support the cost of supporting my blog.
French Onion Tart with Mushrooms
- 1 frozen puff pastry
- 2 teaspoons Dijon mustard
- 2 1/2 pounds yellow onions, about 6 cups when thinly sliced
- 3/4 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 1/2 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 1/4 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.
Welcome to Progressive Eats, our virtual version of a Progressive Dinner Party. This month’s theme is A French Feast, and our host is Coleen who blogs at The Redhead Baker.
If you’re unfamiliar with the concept, a progressive dinner involves going from house to house, enjoying a different course at each location. With Progressive Eats it’s a virtual party. A theme is chosen each month, members share recipes suitable for a delicious meal or party, and you can hop from blog to blog to check them out. Bon appetit!
La boisson (Drink)
- French Connection Cocktail – Creative Culinary
- French-Style Marinated Cheese – Shockingly Delicious
Le pain (Bread)
- Pain Fendu (French Split Bread) – Karen’s Kitchen Stories
Le plat principal (Main Dish)
- Moules à la Normande (Normandy-Style Mussels) – The Redhead Baker
Les plat d’accompagnements (Side Dishes)
- Duck Fat Fries (Pommes Frites) – The Heritage Cook
- Beginner’s French Onion Tart with Mushrooms – The Wimpy Vegetarian
Le dessert (Dessert)
- French Apple Tart – That Skinny Chick Can Bake