Place onions, celery and carrots in deep roasting pan. Pour water over vegetables. Set aside.
Make 10 evenly spaced 2-inch slits on all surfaces of veal leg with thin-bladed knife. Wrap each garlic clove in sage leaf and insert one into each slit. Place veal leg over vegetables in roasting pan.
For Porcini Rub: Combine ingredients such as sugar, 1/4 cup kosher salt, porcini mushrooms, 1 tbsp. white pepper, fennel seeds and red pepper. Reserve 1 Tbsp rub for Sage Sauce. Press remaining rub evenly onto all surfaces of veal leg.
Roast veal leg in 325°F oven until internal temperature reaches 150º F; about 5-1/2 hours. Remove veal from pan; loosely cover and let stand until internal temperature reaches 160º F. Keep warm until ready to use. Strain pan drippings; discard vegetables. Skim fat from drippings. Reserve 1 cup drippings.
For Sage Sauce: While veal is resting, sauté shallots in 2 tbsp butter over medium heat until tender. Add white wine; reduce by half. Add veal stock; reduce by half. Add reserved pan drippings, heavy cream and reserved Porcini Rub; simmer until slightly thickened, skimming sauce occasionally. Thicken with cornstarch slurry, if necessary. Season with 3/4 tsp. salt and 1/4 tsp. pepper.
Whisk in butter; Remove from heat. Add sage and parsley just before serving.
Stand veal leg resting on large end with shank pointing upwards. Carve against the grain downward from the shank to the knuckle; carve around the knuckle and straight down again.
Serve 6 oz veal with 2 to 2-1/2 oz sauce.