There are a few things that can lead to a soggy quiche. First, not cooking all of the water off of the vegetables. Sautéing instead of steaming helps with this but you need to make sure the mushrooms release all of their water and you can squeeze out any excess with a kitchen towel if the spinach releases water at the end. Another culprit is over baking. Quiches are finicky and need to be baked until the center has just set. They will start releasing water when over baked. I recommend using a frozen crust but if you're using a homemade crust, it truly is best to par-bake it. For store-bought refrigerated crusts make sure your crust is still very cold before you bake.