Preheat oven to 375°F
Heat a sauté pan over medium-high heat.
Add the butter and when melted and foaming, add the mushrooms.
Season with salt and pepper and cook until tender. Remove from pan and let cool slightly.
Mix together eggs, milk and cream in a bowl. Season with salt and pepper.
Place spinach in the bottom of the pie shell, top with cheese and mushrooms.
Pour over egg custard mixture and carefully stir to combine ingredients.
Place on a sheet pan and bake for approximately 35 minutes or until firm in the center.