5 Ingredient Onion Tart with Mushrooms Recipe

Step-by-step instructions for making a French onion tart with photos. Mushrooms and tomatoes are added for extra umami. This makes a perfect fall supper or party appetizer.

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5 from 15 votes

5 Ingredient Onion Tart with Mushrooms Recipe

Prep Time30

Cook Time1 20

Total Time1 50

Course: Appetizer, Vegetable Main or Side

Cuisine: French

Keyword: Caramelized Onions, French Onion Tart, Onion Tart

Servings:

Calories: 541.9kcal

Author: Susan Pridmore

Equipment

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.

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