Preheat oven to 400F. If your crust is homemade, start by blind baking it. This is the process of partially baking a crust before filling it. This ensures that the pie shell gets baked all the way through, and prevents the bottom of the crust from getting soggy. Transfer the dough to your pie dish and pricking the bottom of the crust with a fork. Next, line the inside of the crust with parchment paper and pie weights or dried beans. Bake for 20 minutes. If your crust is store bought, follow the directions on the package.
While the crust is baking, heat butter in a skillet. Sautee the onion until browned, then add in the mushrooms and cook an additional 5 minutes. Add in the spinach and cook until wilted. Remove the crust from the oven and take the parchment paper and weights or beans out. Let the crust cool for a few minutes then place the veggie mixture in it. Whisk together the heavy cream, milk, eggs and spices. It's best to do this with an immersion blender if you have one. Pour it over the veggie mixture.
Lower the oven to 350F. Bake for 40-50 minutes, until the quiche is puffed and golden.
Serve warm.
