Directions

1. Prepare the Tart Crust:

   In a food processor, combine the flour and salt. Add the cold, cubed butter and pulse until the mixture resembles coarse crumbs.

   While pulsing, drizzle in the ice water, one tablespoon at a time, until the dough just comes together.

   Turn the dough out onto a floured surface and shape it into a disc. Wrap in plastic wrap and chill in the refrigerator for at least 30 minutes.

2. Caramelize the Onions:

   In a large skillet, melt the butter and olive oil over medium-low heat.

   Add the thinly sliced onions and cook, stirring occasionally, until they become soft and caramelized, about 30-40 minutes.

   Optional: Sprinkle brown sugar over the onions in the last 10 minutes to enhance caramelization.

   Season with salt and pepper, then set aside.

3. Prepare the Mushroom Filling:

   In a separate pan, melt the butter and olive oil over medium heat.

   Add the minced garlic and thyme leaves, and sauté for about 1 minute until fragrant.

   Add the sliced mushrooms and sauté for 8-10 minutes, or until they are tender and any liquid released has evaporated.

   Season with salt and pepper to taste, then set aside.

4. Roll Out the Tart Crust:

   Preheat your oven to 375°F (190°C).

   On a floured surface, roll out the chilled tart dough to fit a 9-inch tart pan. Press the dough into the pan, trimming any excess.

5. Assemble the Mushroom Tart:

   Spread the caramelized onions evenly over the tart crust.

   Add the sautéed mushrooms on top of the onions.

   Sprinkle the grated Gruyère cheese over the mushrooms.

   Fold any overhanging pastry back over the filling, or create a decorative edge.

6. Bake and Serve:

   Brush the exposed pastry edges with the beaten egg wash for a golden finish.

   Bake the tart in the preheated oven for 25-30 minutes or until the crust is golden brown and the cheese is bubbly.

   Remove from the oven and let it cool for a few minutes before slicing.

   Sprinkle freshly chopped parsley over the tart for a burst of color and flavor.

   Slice and serve your mushroom tart while it's still warm. It's delicious on its own or alongside a crisp green salad.