Preheat your oven to 400°F (200°C). Roll out the puff pastry on a floured surface until about 1/8 inch thick. Fit the pastry into a tart pan and prick the bottom with a fork. Set aside.

In a skillet, melt butter over medium heat. Add leeks and mushrooms, sauté for 8-10 minutes until softened.

In a bowl, whisk together eggs and cream until smooth. Fold in sautéed vegetables and season with salt and pepper.

Pour the filling into the pastry crust. Bake for 25-30 minutes until the filling is set and crust is golden brown.

Allow the tart to cool for 10 minutes before slicing. Garnish with fresh herbs if desired and serve.