Roll out your single-crust pie dough on a floured surface to about 1/8-inch thickness. Fit the dough into a 9-inch pie plate, trimming excess, and chill in the refrigerator while preparing the filling.

In a skillet over medium heat, melt 1 tablespoon of butter. Add sliced leek and sauté for 2-3 minutes until tender. Set aside to cool.

In the same skillet, add remaining butter over medium-high. Sauté your choice of mushrooms for 4-6 minutes until tender. Combine cooked leeks with mushrooms, adding salt and pepper. Let cool.

Preheat oven to 375°F (190°C). Spread half of the shredded cheddar cheese in the chilled crust. Layer sautéed mushroom and leek mixture, then top with remaining cheese.

In a mixing bowl, whisk together eggs and heavy whipping cream. Pour custard mixture evenly over the pie filling.

Bake pie in preheated oven for 30-35 minutes until set and lightly golden. Check doneness by inserting a knife; it should come out clean.

Allow pie to cool for a few minutes before slicing. Garnish with minced parsley if desired.