Combine the butter, cream cheese, and 1 cup of the flour in a small bowl. Mix with a pastry blender or fork, cutting the butter and cream cheese into the flour until a consistently crumbly mixture is formed.
Place the flour mixture in a 10" pie plate or quiche pan, and press evenly to form a crust. Refrigerate. Pre-heat the oven to 350°F.
Cook the spinach in a pot of lightly salted boiling water, drain well, and chop. Drain the spinach again on paper towels to remove all excess moisture.
In a large bowl, combine the spinach with the two tablespoons flour. Add the cheddar and Swiss cheeses, eggs, mayonnaise, milk, bacon bits or crumbled bacon, mushrooms, and green onions.
Pour the mixture into the chilled pastry shell and bake for 1 hour, or until set. Serve warm.