In a large stockpot on medium-high heat, saute the onions with the butter and bay leaf for 20 minutes, until the onions turn a rich golden brown color. Deglaze the pan with the vermouth and cognac and simmer uncovered for 5 minutes. Add the white wine and simmer uncovered for 15 more minutes. Add the veal stock, salt and pepper. Bring to a boil, then simmer uncovered for 20 minutes. Remove the bay leaf, taste for salt and pepper, and serve hot with a crostini floating on top, grated Parmesan and a slice of provolone. Melt the cheese under the broiler until hot and bubbly.