Slice the onions thinly and cook them in the butter and olive oil over low heat for 15 minutes.
Add salt and sugar, stirring.
Simmer for 40 minutes, stirring occasionally.
Stir the bouillon base into the hot water along with the mushrooms.
Let rest for 30 minutes to rehydrate the mushrooms.
Stir the rosemary and flour into the pot, until thick.
Add the mushrooms, broth, wine, and pepper. Stirring and simmering for one hour.
Preheat the oven to 325F.
Pour the soup into oven-safe bowls.
Slice the bread and butter each slice.
Arrange the butter bread on top of the soup and top with a generous amount of grated cheese.
Bake for 20 minutes and finish off with roughly 2 minutes on broil.
Top with grated raw onion or onion greens.