In a Dutch oven or soup kettle, melt butter. Add onions and sugar; cook over low heat until lightly browned, about 1 hour.
Sprinkle flour over onions and stir until blended. Gradually stir in broth. Add the water, salt, dried onion, bouillon, garlic salt and pepper.
Bring to a boil; cook and stir for 2 minutes. Reduce heat; cover and simmer for 45 minutes.
Ladle soup into ovenproof bowls. Top with a slice of toasted bread; sprinkle with cheese. Place on a baking sheet. Bake at 400° for 5 minutes.
TEST KITCHEN TIP Refrigerate the onions before slicing—your tear ducts won't get as much of a workout!
Every editorial product is independently selected, though we may be compensated or receive an affiliate commission if you buy something through our links. Ratings and prices are accurate and items are in stock as of time of publication.