Heat oven to 350°F. In 2-quart saucepan, melt butter over medium-high heat. Cook onion and garlic in butter 3 to 4 minutes, stirring frequently, until softened. Stir in flour until all flour is blended with butter. Stir in broth and sour cream until smooth. Heat to boiling; cook about 2 minutes, stirring constantly, until sauce is thickened. Remove from heat. Stir in cheese until melted. Stir in chiles.