Parboil spinach for 1-2 minutes; drain and squeeze out all the liquid. Mix spinach, 2 cups Cheddar cheese and onion in the large pie plate. Beat milk, eggs, baking mix, salt and pepper until smooth (15 seconds in blender or 1 minute by hand). Pour into pie plate and carefully mix into spinach mix.
Add the cup of Swiss cheese covering completely. If using cheddar, add the last 15 minutes. Bake at 350 degrees until done. (Bake until knife inserted into the center comes out clean-20 to 30 minutes) depending on your oven.
Let stand at least 5 minutes before cutting.
