Preheat oven to 225 degrees. Make a bouquet garni by wrapping the parsley sprigs, bay leaves, leek leaves, whole peppercorns and cloves in cheesecloth and tying it with kitchen twine. Make beurre manie by kneading the softened butter and flour together with your fingertips until you have a uniform paste.
Blanch the veal chunks in water. In a large roaster combine blanched veal, bouquet garni, onions, and carrots. Cover with stock. Bring up to a simmer and place roaster in the oven. Braise until veal is completely tender, about 3 hours. Add leeks cook for another 30 minutes. Strain the stock from the meat and vegetables, discard the bouquet garni. In a medium pot, reduce stock by half. Whisk in beurre manie to thicken. Add the creme fraiche, and season with salt and pepper. Pour the thickened sauce over the meat and vegetables. Garnish with chervil and parsley.