Savory Mushroom Onion Tart (Printable)

Flaky pastry filled with caramelized onions, mushrooms, and creamy cheese for a savory, elegant dish.

# What You'll Need:

→ Pastry

01 - 1 sheet ready-made puff pastry, thawed (approximately 8.8 oz)

→ Vegetables

02 - 2 tablespoons olive oil
03 - 1 tablespoon unsalted butter
04 - 2 large yellow onions, thinly sliced
05 - 14 oz cremini or button mushrooms, sliced
06 - 2 cloves garlic, minced

→ Filling

07 - 3 large eggs
08 - ⅔ cup heavy cream
09 - 1 cup grated Gruyère or Swiss cheese
10 - 1 tablespoon fresh thyme leaves or 1 teaspoon dried thyme
11 - 1 teaspoon salt
12 - ½ teaspoon black pepper

→ Garnish

13 - Fresh chives or parsley, chopped (optional)

# How to Make It:

01 - Preheat oven to 400°F. Line a 9-inch tart pan with parchment paper.
02 - Roll out the puff pastry and press it into the prepared tart pan. Trim excess pastry and prick the base with a fork. Chill in the refrigerator while preparing the filling.
03 - Heat olive oil and butter in a large skillet over medium heat. Add sliced onions and cook, stirring occasionally, until soft and golden, about 12 minutes.
04 - Add sliced mushrooms to the skillet and continue cooking for 8 to 10 minutes until mushrooms release moisture and turn golden. Stir in minced garlic and thyme leaves, cook for 1 additional minute. Remove from heat and allow to cool slightly.
05 - In a mixing bowl, whisk together eggs, heavy cream, salt, and black pepper. Fold in grated cheese until combined.
06 - Evenly spread the onion and mushroom mixture over the chilled pastry shell. Pour the egg and cheese custard evenly on top.
07 - Bake in the preheated oven for 25 to 30 minutes, or until the filling is set and the pastry is golden brown.
08 - Let the tart cool for 10 minutes before garnishing with fresh herbs. Slice and serve warm or at room temperature.

# Expert Advice:

01 -
  • The puff pastry does nearly all the work while you get credit for looking like a French bakery.
  • It's genuinely impressive enough for guests but honest enough to eat straight from the oven in your kitchen.
  • Caramelized onions are a flavor investment that pays dividends in almost everything you make afterward.
02 -
  • Don't skip the chilling step for the pastry or rush the caramelizing of the onions—both are where the quality of this dish lives.
  • If your filling looks slightly wet when you pull it from the oven, that's actually okay; it will set as it cools, and overbaking leads to a rubbery, sad custard.
  • The tart tastes even better the next day when all the flavors have settled together, so don't hesitate to make it ahead.
03 -
  • Keep your pastry cold from the moment you take it out of the freezer until it goes into the oven—this is what creates those beautiful, crispy layers.
  • Taste your filling mixture before you pour it into the shell; season it generously because mushrooms and onions absorb salt differently than other vegetables.

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