swedish meatball recipe

1 pound ground beef - 1/2 pound ground pork - 1 small onion, finely chopped - 1/2 cup breadcrumbs - 1/4 cup milk - 1 egg - 1 teaspoon salt - 1/2 teaspoon black pepper - 1/4 teaspoon ground allspice - 1/4 teaspoon ground nutmeg - 2 tablespoons butter (for frying)