Combine onions, broth, consomme, and soup mix in a slow cooker. Cook on HIGH setting for 4 hours, or LOW setting for 8 hours.
Ladle soup into serving bowls. Top with a slice of French bread. Sprinkle 2 tablespoons cheese over each bread slice. Put soup bowls under broiler until cheese is melted. Alternatively, place the bread on a baking sheet and top with cheese; broil until cheese has melted, about 30 to 40 seconds, and set on top just before serving.