Melt butter in a large saucepan over medium heat. Add onion, and sauté 3 minutes. Stir in flour, salt, and pepper. Gradually add milk, stirring with a whisk until well blended. Bring mixture to a simmer; cook 1 minute or until slightly thick, stirring constantly. Remove saucepan from heat; stir in rice, feta cheese, and spinach. Add egg and egg white, stirring until blended.
Pour spinach mixture into a 9-inch pie plate coated with cooking spray. Sprinkle Parmesan cheese over pie. Bake at 400° for 35 minutes or until golden.