Preheat your oven to 375°F (190°C).
2. Caramelize Onions
In a large skillet, heat olive oil and butter over medium heat. Add sliced onions, sugar, salt, and pepper. Cook slowly, stirring occasionally, until the onions are deeply caramelized, about 30–40 minutes. Remove from heat and set aside.
3. Cook Mushrooms
In the same skillet, add mushrooms and garlic. Cook until mushrooms are golden and have released their moisture, about 10 minutes. Stir in thyme and cook for another minute. Remove from heat.
4. Prepare Tart Crust
Roll out the tart crust and fit it into a tart pan. Trim the edges as needed and prick the bottom with a fork. Blind bake for 10 minutes, then remove from the oven.
5. Assemble Tart
Spread the caramelized onions evenly over the bottom of the crust. Layer the cooked mushrooms on top, then sprinkle with grated Gruyere cheese.
6. Prepare Custard
In a small bowl, whisk together the heavy cream and eggs until smooth. Pour the mixture evenly over the tart filling.
7. Bake
Bake in the preheated oven for 25–30 minutes, or until the custard is set and the top is golden brown. Allow the tart to cool slightly before slicing.
8. Serve
Serve warm with a fresh green salad and a glass of your favorite wine.
Tips & Variations
Cheese Swap: Try Swiss, Comté, or Fontina instead of Gruyere for a different flavor.
Add Veggies: Include sautéed spinach or roasted red peppers for added color and nutrients.
Make Ahead: Caramelize onions and cook mushrooms a day ahead for quicker assembly.
Nutritional Info (per serving, serves 6)
Calories: 320 kcal
Protein: 12 g
Carbs: 28 g
Fat: 18 g
