1. Heat oven to 400°F.
2. Lightly grease pie plate. Mix spinach, onion, and cheese in pie plate.
3. Beat remaining ingredients until smooth (15 seconds on high in blender or 1 minute if using hand beater). Pour into pie plate.
4. Bake about 35 minutes or until golden brown and knife inserted halfway between edge and center comes out clean.
5. Let stand 5 minutes before serving.