In 5-quart Dutch oven, melt butter over medium-high heat until sizzling. Cook onions in butter 13 to 15 minutes, stirring frequently, until onions are light golden brown. Stir in garlic; cook 30 seconds. Sprinkle flour on onion mixture; cook and stir constantly 1 minute. Slowly stir in chicken stock. Heat to boiling; reduce heat to low. Cook uncovered 10 minutes, stirring occasionally.