- Homemade or store-bought pie crust
- 2 Tbsp extra-virgin olive oil
- 1 medium red onion, halved and thinly sliced
- Salt and black pepper
- 1¼ lb. thinly sliced cremini mushrooms (approx. 8 cups)
- 1 Tbsp fresh thyme leaves (from 4 sprigs fresh thyme)
- 2 large eggs
- ⅓ cup heavy cream
- 5 oz. Parmesan, grated
- 3 oz. gruyere, grated
Heat 1 Tbsp olive oil in a large skillet over medium heat. Add onion, sprinkle with salt, and cook, stirring occasionally, until the onion is translucent and slightly browned, about 5 minutes. Transfer to a large mixing bowl. Add the remaining 1 Tbsp olive oil to the skillet and heat over medium. Add the mushrooms and thyme, season with salt and pepper, and cook, stirring occasionally, until mushrooms are tender and their liquid has evaporated, 8-12 minutes. Transfer to the bowl with the onion and let cool completely.
Take chilled (but not frozen) dough and roll it out on a floured work surface into an 11-inch circle. Fit the crust into a 9-inch tart pan with a removable bottom (or a pie pan), trimming off or crimping any extra dough around the perimeter. Freeze the crust in the pan for 15 minutes or refrigerate for 1 hour.
Heat oven to 375 F. In a small bowl, whisk together eggs and cream; season with salt and pepper. Pour into the bowl with the mushrooms, add cheese and stir until combined. Transfer the filling into the prepared tart pan (or pie tin) and spread it into an even layer using a rubber spatula. Bake tart for about 40 minutes, until the crust and filling are golden-brown. Let cool for 10 minutes in the pan, then serve hot.