Savory Mushroom Onion Tart (Printable Version)

Golden tart featuring caramelized onions, mushrooms, creamy cheese in a flaky crust.

# What You'll Need:

→ Pastry

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

→ 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 - 2/3 cup heavy cream
09 - 1 cup grated Gruyère or Swiss cheese
10 - 1 tablespoon fresh thyme leaves or 1 teaspoon dried
11 - 1 teaspoon salt
12 - 1/2 teaspoon black pepper

→ Garnish

13 - Fresh chives or parsley, chopped (optional)

# Directions:

01 - Preheat oven to 400°F. Line a 9-inch tart pan with parchment paper.
02 - Roll out puff pastry and press into the prepared tart pan. Trim excess and prick base with a fork. Chill in refrigerator while preparing 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 mushrooms to skillet and cook for 8 to 10 minutes until moisture evaporates and mushrooms turn golden. Stir in minced garlic and thyme; cook another minute. Remove from heat and let cool slightly.
05 - In a bowl, whisk eggs, heavy cream, salt, and pepper. Fold in grated cheese until combined.
06 - Evenly spread the onion and mushroom mixture over chilled pastry. Pour egg and cheese mixture on top.
07 - Bake for 25 to 30 minutes until filling is set and pastry is golden brown.
08 - Allow tart to cool for 10 minutes before garnishing with fresh chives or parsley. Serve warm or at room temperature.

# Expert Tips:

01 -
  • The pastry turns impossibly golden and crispy while the filling stays silky and rich—it's one of those dishes where every element actually cooperates.
  • It works equally well as a fancy dinner or casual lunch, warm or room temperature, which means you can make it ahead and stop stressing.
  • Those caramelized onions taste like they've been cooking for hours, but the technique is surprisingly straightforward once you know the trick.
02 -
  • Don't skip chilling the pastry shell before filling it—this is what keeps the bottom from turning soggy and is honestly the difference between a good tart and a great one.
  • If your mushrooms release a lot of liquid as they cook, don't pour it all into the tart or the custard will be watery; instead, let them cook uncovered so the moisture evaporates and concentrates their flavor.
  • The key to caramelizing onions properly is medium heat and patience—high heat makes them brown too quickly and taste bitter, which ruins the whole dish.
03 -
  • Invest in a good tart pan with a removable bottom—it makes slicing and serving so much easier, and your tart will look restaurant-quality without the restaurant-level stress.
  • Let your puff pastry thaw completely but keep it cold—the contrast between the room-temperature work surface and the cold pastry helps it puff up and crisp up beautifully in the oven.

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