Preheat oven to 400°.
Melt margarine in large saucepan over medium heat. Add chopped onion, and saute 3 minutes. Stir in flour, salt, and pepper. Gradually add milk, stirring with a whisk until well-blended. Bring mixture to a simmer, and cook 1 minute or until slightly thick, stirring constantly. Remove saucepan from heat, and stir in the cooked rice, crumbled feta cheese, egg, egg whites, and spinach.
Pour mixture into a 9-inch pie plate coated with cooking spray. Sprinkle Parmesan cheese over pie. Cover and chill.
To serve, let stand 30 minutes at room temperature.
Bake pie at 400° for 35 minutes or until set. Broil 2 minutes or until pie is golden brown.
Note: Besides the nutrients listed below, each serving provides 3,909 IUs vitamin A and 13mg vitamin C.