Perfect Gluten Free Quiche

For the crust 1/2 recipe gluten free pie crust or extra flaky gluten free pie crust, chilled

For the filling 2 tablespoons (28 g) unsalted butter

extra virgin olive oil 

small yellow onion, peeled and diced

white or baby portobello mushrooms, sliced*

kosher salt

freshly ground black pepper

eggs (200 g, weighed out of shell)

half and half, or 1 cup (8 fluid ounces) whole milk

ounces Gruyère cheese (or other semi-hard cheese, like Asiago, Jarlsberg or sharp white cheddar), grated

*Mushrooms can be replaced with 10 ounces broccoli or cauliflower florets, blanched and blotted dry, or 1 pound frozen cut spinach, thawed and squeezed dry.