CREAMY FOUR-CHEESE MACARONI

1/3 cup all-purpose flour
2 2/3 cups 1% low-fat milk
3/4 cup shredded fontina cheese or Swiss cheese
1.2 cup grated fresh Parmesan cheese
1/2 cup shredded extra-sharp cheddar cheese
3 ounces light processed cheese 
6 cups cooked elbow macaroni (about 3 cups uncooked)
1/4 teaspoon salt
Cooking spray
1/3 cup crushed onion melba toasts (about 12 pieces)
1 tablespoon butter or stick margarine, softened

Preheat oven to 375°F.

Lightly spoon flour into a dry measuring cup, and level with a knife. Place the flour in a large saucepan. Gradually add the milk, stirring with a whisk until blended. Cook over medium heat until thick (about 8 minutes), stirring constantly. Add cheeses, and cook 3 minutes or until cheeses melt, stirring frequently. Remove from heat, and stir in macaroni and salt.