Place the meat in mixing bowl and add the egg. In a smaller bowl mix bread crumbs, 1/4 cup of yogurt and grated onion. Add this to the meat mixture. Add salt, pepper and nutmeg. Mix well with your hands, but do not over mix. Shape the mixture in to balls about 11/3 inches in diameter. Sprinkle the pan with paprika and roll the meatballs turning gently to coat well. Heat the butter in a large non-stick skillet and cook the meatballs turning gently and shaking the pan until they are nicely browned. Scatter the mushrooms, onion and garlic around the meatballs. Shake the skillet to distribute the ingredients evenly. Cover and simmer for about five minutes. Shape the skilett a few times. In a small bowl mix the sherry and tomato paste. Blend well. Add to the skillet, cover and simmer for 10 minutes shaking often. Stir in the remaining yogurt and bring to just a simmer. Do not over boil. Check the seasoning. Sprinkle with parsely and serve piping hot with buttered fine noodles.