Simple, but flavorful. My husband pulled it from a cookbook we picked up on our honeymoon (pre-Katrina New Orleans). It originally called for red onion, and, sure, the first few bites were a flavor explosion, but then it just became too much. You like that? Use reds. You like a milder onion soup? Use Vidalia's. Etc.
Cut onion rather small. Fry very slowly in oil and butter until soft and translucent. Add sugar, salt, and cayenne. Put all in casserole and add stock. After toasting French bread, float it on top of soup. Sprinkle cheese over all. Bake in oven for 20 minutes.