Preheat oven to 375°F (190°C).
Place the pie crust into a 9-inch pie dish. Prick the bottom with a fork.
Sauté ½ cup chopped mushrooms (or your chosen veggie) in a little oil until soft. Let cool slightly.
In a bowl, whisk together 4 eggs, 1 cup heavy cream (or milk), salt, pepper, and a pinch of nutmeg.
Spread the sautéed vegetables, 1 cup shredded cheese, and ¼ cup chopped onion evenly in the crust.
Pour the egg mixture over the filling.
Bake for 35–40 minutes, or until the center is set and the top is lightly golden.
Cool for 5–10 minutes before slicing. Serve warm or at room temperature.