Heat oven to 350 degrees. Spray 2-qt. Casserole with nonstick cooking spray. Melt margarine in 3-qt. Saucepan over low heat. Cook flour in margarine, stirring constantly, until thickened; remove from heat.
Stir milk into flour mixture. Heat to boiling, stirring constantly with wire whisk. Boil and stir 1 minute. Stir in remaining ingredients. Spoon into casserole. Bake uncovered 30-35 minutes or until hot and bubbly.