Melt 2 tablespoons butter in heavy medium skillet over medium heat. Add onion and sauté until translucent, about 5 minutes. Cool slightly. Combine bread and stock in large bowl. Mix in onion mixture, egg, allspice and pepper. Add ground beef and veal and blend well. (Can be prepared 3 hours ahead. Cover and chill.)Shape meat into 1 1/4-inch balls. Melt remaining 2 tablespoons butter in heavy large skillet over medium heat. Add meatballs and sauté until cooked through and brown, turning occasionally, about 20 minutes. Using slotted spoon, transfer meatballs to platter and keep warm. Reserve drippings in skillet.
Add flour to drippings in skillet and stir over medium heat until brown, about 4 minutes. Gradually whisk in stock and cream. Simmer until sauce is thick and smooth, stirring frequently, about 5 minutes. Season to taste with allspice, salt and pepper. Pour sauce over meatballs and serve.