Meanwhile, heat oven to 350°F. Spray 2-quart casserole with nonstick cooking spray. Melt margarine in medium saucepan over medium heat. Add onions and garlic; cook 1 minute. Stir in flour; cook until mixture is smooth and bubbly. Gradually add milk, stirring constantly, until mixture boils and thickens. Add cheese; stir until melted.