French Onion and Mushroom Tart with Gruyere Cheese

French Onion and Mushroom Tart with Gruyere Cheese

 

Total Time: 55 minutes Yield: #

A golden, flaky tart filled with sweet caramelized onions, earthy mushrooms, and nutty Gruyere this dish brings French bistro elegance straight to your kitchen. Perfect for brunch, a light dinner, or a show-stopping appetizer.

Ingredients

    12 oz shiitake or cremini mushrooms (or a mix), cleaned and sliced
    1 sheet puff pastry (14 oz), thawed
    2 medium yellow onions, thinly sliced
    1¼ cups Gruyere cheese, grated
    ¼ cup beef stock (substitute vegetable stock for vegetarian version)
    ¼ cup dry sherry
    3 tbsp ghee or unsalted butter, divided
    2 cloves garlic, minced
    1 tbsp fresh thyme leaves, divided
    1 large egg, whisked (for egg wash)
    Salt and freshly ground black pepper, to taste
    Parchment paper, for baking

Instructions

Step 1: Caramelize the Onions
Start by creating the flavor base of the tart—the caramelized onions.
In a large skillet, heat 1½ tablespoons of ghee over medium heat.
Add the thinly sliced onions, season with salt and pepper, and sauté.
Stir frequently to prevent sticking or burning. Allow the onions to slowly cook for about 20 minutes, until they become soft and golden brown.
Pour in the ¼ cup of sherry, and let it bubble away, scraping up any fond from the bottom of the pan.
Then, add the ¼ cup of beef stock. Let the mixture reduce until nearly all the liquid has evaporated, creating a sticky, deeply flavorful onion compote.
Remove from heat and set aside.

Step 2: Sauté the Mushrooms
Using the same skillet for depth of flavor:
Heat the remaining 1½ tablespoons of ghee over medium-high heat.
Add the sliced mushrooms. Season with salt and black pepper.
Let the mushrooms cook undisturbed for the first couple of minutes so they get golden brown. Then stir occasionally for about 5–7 minutes.
Add in the minced garlic and 1 teaspoon of fresh thyme leaves. Cook for another 30 seconds until fragrant.
Transfer the mushroom mixture to a paper towel-lined plate to drain any excess moisture and cool.

Step 3: Prepare the Puff Pastry
Preheat your oven to 400°F (200°C).
Unfold the thawed puff pastry on a piece of parchment paper.
Roll it out gently to about 10×13 inches, ensuring it’s roughly ⅛ inch thick.
Score a ½-inch border around the edges with a knife (don’t cut all the way through) to allow a crust to rise.
Use a fork to prick the center of the pastry all over—this prevents bubbling in the middle while baking.

Step 4: Assemble the Tart
Sprinkle ¾ cup of Gruyere cheese evenly over the center of the pastry, keeping it within the scored border.
Spoon the caramelized onions over the cheese, spreading them evenly.
Next, layer on the sautéed mushrooms, making sure they’re evenly distributed.
Top with the remaining ½ cup of Gruyere cheese.
Brush the outer edges of the pastry with the whisked egg—this gives the crust its beautiful golden shine.

Step 5: Bake to Perfection
Transfer the parchment and assembled tart to a baking sheet.
Bake in the preheated oven for 26–30 minutes, or until the pastry is golden brown, puffed at the edges, and the cheese is bubbling and lightly browned.
Remove from the oven and let the tart rest for 10 minutes.
Sprinkle the top with the remaining thyme leaves for a fresh, herbal finish.

Notes

For deeper flavor, caramelize the onions until they’re a rich, golden-brown color. Don’t rush this process—it’s worth the time.
Make sure your mushrooms release their moisture and brown properly. This prevents the tart from becoming soggy.
Always thaw puff pastry in the fridge (not on the counter) for best texture.