Saute minced onion and garlic in butter in large heavy saucepan. Blend in flour with wooden spoon. Add milk and cheese. Stir constantly until cheese is melted and mixture is smooth and thick. If mixture seems too thick to use as a dip, blend in a little water. Mix in the chopped green chile (fresh, frozen or canned) to suit your taste. Serve in a chafing dish with tostados, corn chips or raw vegetable sticks to dip in the mixture.