1. Heat butter in large pot or Dutch oven over medium-low heat. Add onions and cook, stirring occasionally until golden brown, about 40 minutes.
2. Sprinkle onions with flour, and salt; cook, stirring, 3 minutes. Slowly add beef broth, increase heat to medium-high and bring soup to a boil. Reduce heat and simmer for 15 minutes.
3. Preheat broiler to high-heat. Place rack 4 inches from heat source. Divide soup among 6 oven safe serving bowls. Top each with 2 slices of hard roll. Sprinkle with cheese and broil 3 minutes or until cheese is melted and bubbling.