Vintage Recipe--Onion Soup

2 Tbsp butter
1 tsp sugar
1/4 tsp dry mustard
4 cups hot water
6 beef bouillon cubes
4 slices buttered french bread
1/4 cup grated parmesan cheese

Melt butter in a large saucepan; blend in sugar and mustard. Add onion; cook over low heat, stirring frequently, about 15 minutes, or until tender but not brown. Pour in water; bring to boiling. Blend in and dissolve beef bouillon cubes. Simmer, uncovered, about 15 minutes. Pour into individual heat-proof bowls. top with slices of bread; sprinkle with cheese. Broil, 3 inches from the heat, 2 to 3 minutes, or until cheese is golden brown.