Meanwhile, in 4- to 5-quart Dutch oven, melt 3 tablespoons butter over medium heat. Add onions, salt and pepper flakes. Cook 7 to 9 minutes, stirring frequently, until lightly browned. Add flour; stir 2 to 3 minutes or until lightly browned. Slowly stir in stock and cream. Heat to simmering; cook 2 to 3 minutes or until very thick.