When the onions are soft, increase the heat slightly and add the sugar. Stir, then add the sifted flour while continuing to stir. When the mixture thickens, pour in the broth. Continue cooking for 30 minutes.

Serve with toasted bread croutons and grated cheese.

Note: For a French-style version, ladle the soup into oven-safe bowls, add slices of baguette, and sprinkle with grated Emmental cheese. Bake until a golden crust forms.