Cut the onions into thin round slices and saute them in butter on medium heat. While stirring, add the finely chopped leeks, the crushed garlic, broth cubes and 4 1/5 cups (1 liter) of water.
Season the soup with thyme, salt and pepper to taste. Let it boil on medium heat for about 20-25 minutes. Near the end of cooking, add the white wine. Put the soup in the oven in a suitable container, cover the top with lightly fried cubes of bread.
Add grated emmental cheese on top of the cubes and bake for about 10 minutes at 536°F (280 °C) until the top is lightly browned. Serve the soup hot.