Heat oven to 350°F. Spray 13x9-inch baking dish with cooking spray. Spray 1 1/2-quart saucepan with cooking spray; heat over medium-high heat. Cook and stir finely chopped onion in saucepan about 3 minutes or until almost tender; add garlic. Cook and stir about 30 seconds longer or until fragrant. Stir in flour. Cook and stir 1 minute (mixture will be dry). Gradually add milk, stirring with whisk; heat to boiling. Cook and stir about 2 minutes or until thickened. Remove from heat. Stir in remaining 1/4 cup cheese, the salt and pepper.