Pat brisket with paper towels to dry. Heat oil in casserole. Season meat with salt and pepper. Brown meat on all sides; remove from casserole. Add onions to casserole and saute until golden. Return meat to casserole, and add garlic and peppercorns. Cover and simmer 3 hours, or place in a preheated 325 degree oven for 3 hours.
Season with salt and pepper, slice and serve with its own juice. If a thicker sauce is desired, mix 1 tablespoon potato starch with 1 tablespoon water; add to sauce, stirring and heat through. Sprinkle with chopped parsley, Serve with white horseradish.