Melt the butter in the 6-quart pot and add the olive oil. Add the onions and cook covered over medium heat until the onions are tender and translucent, about 10 to 15 minutes.
Stir in the kosher salt and sugar and raise the heat to medium high. Cook uncovered until the onions are browned, about 25 to 30 minutes.
Add the flour and cook about 3 to 4 minutes. Remove from heat and add 2 cups of heated vegetable stock and whisk it in. Add the remaining stock, the Cognac, and the vermouth, and simmer partially covered over low heat for 1-1/2 hours.
To serve: Preheat the broiler to 500° F. Pour the soup into the oven-proof bowl or crocks and lay a piece of the sliced baguette on top of each. Add a piece of sliced Swiss and sprinkle on some grated Parmesan. Put the crocks on the foil-lined baking sheet. Broil until the cheese it bubbly and golden brown. Serve hot with a big side salad or some roasted broccoli.