Start by rolling out your pie crust on a lightly floured surface until it's about 1/8 inch thick. Carefully fit the dough into a tart pan, pressing it into the edges. Trim any excess crust and use a fork to poke small holes in the base. Refrigerate the crust for about 15 minutes to firm up while you prepare the filling.

In a large skillet, melt 2 tablespoons of butter over medium heat. Add the sliced leeks and mushrooms, stirring frequently. Cook for about 5-7 minutes until the leeks are tender and the mushrooms are browned, releasing their earthy aroma. Season with a pinch of salt and pepper to enhance the flavors, then remove from heat and allow to cool slightly.

In a mixing bowl, whisk together 3 large eggs and 1 cup of cream until well combined. Once the sautéed leeks and mushrooms have cooled a bit, gently fold them into the egg mixture, ensuring everything is evenly distributed.

Carefully pour the leek and mushroom mixture into the chilled pie crust, spreading it evenly with a spatula.

Preheat your oven to 375°F (190°C). Once heated, place your tart on the middle rack and bake for approximately 30-35 minutes.

Once baked, remove the tart from the oven and allow it to cool for about 10 minutes. Serve warm or at room temperature.