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.
For the veal stock
Roast the bones slowly over night in a 275 degree oven until the are brown and caramelized. Add the bones to a stock pot and cover with water. Add carrots, onions and celery ,bay leaves, tomato paste and wine. Simmer on low for 48 hours. Continue to add water to cover the bones. Strain through a fine sieve. Refrigerate until ready to use.