In 2-quart saucepan, melt butter over medium heat. Add onion and garlic; cook about 3 minutes or until softened. Sprinkle in flour. Cook and stir 1 minute. With whisk, gradually beat in milk, cooking 5 to 7 minutes or until mixture is slightly thickened. Stir in enchilada sauce. Add cheese; cook until cheese is melted, stirring occasionally.