5 Ingredient Onion Tart with Mushrooms Recipe
Servings: 4
Ingredients
- 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)
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 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.