Thinly slice the yellow onions.

Saute the sliced onions in butter slowly in a covered pan.

Cook the onions on low heat for 15 minutes, ensuring they do not brown.

Add salt, sugar, and flour to the onions, stirring well to combine.

Transfer the onion mixture to a crock-pot.

Add the beef bouillon to the crock-pot.

Cook on low heat for 6 to 8 hours or on high heat for 3 hours.

Place croutons or French bread in individual bowls.

Ladle the soup into the bowls over the croutons.

Top each bowl with grated Mozzarella and Parmesan cheeses.

Broil the bowls briefly until the cheese is melted and lightly browned.