Peel the onions, cut in half and cut into thin slices.
In a large, heavy saucepan heat the oil and butter. Add onions and stir well. Cook on low heat, covered, 10 minutes then remove the cover.
Add salt and sugar and fry 30 minutes, stirring constantly. At the end of cooking the onions should be deep golden brown.
Sprinkle onions with flour and stir well. Saute 3 minutes. Meanwhile, heat the beef stock separately.
Deglaze the saucepan with cognac and vermouth. Add the stock and mix everything well. Simmer 90 minutes, loosely covered. Season with salt and pepper.
Toast the baguette under the grill until golden brown on both sides. Lay a slice of cheese on each baguette and return to the grill to melt the cheese.
Lay the baguettes on top of the soup in the saucepan and serve!