Preheat your oven to 400°F (200°C). Roll out the puff pastry until about 1/8 inch thick, fit into a tart pan, and chill for 30 minutes.

In a skillet, melt 2 tablespoons of butter and sauté 2 sliced leeks and 8 ounces of mushrooms for about 8 minutes.

In a mixing bowl, whisk together 3 beaten eggs, 8 ounces cream cheese, and 1 cup grated cheese until smooth.

Fold the sautéed leeks and mushrooms into the egg and cheese mixture, ensuring even distribution.

Pour the mixture into the chilled tart crust, leaving some space at the top.

Bake in the preheated oven for 35-40 minutes, until the filling is bubbling and the crust is golden-brown.

Let the tart cool for 10-15 minutes before slicing and serve warm or at room temperature.