In a large skillet heat 1 tablespoon of butter and add in diced onions.While the onions are sweating, combine bread crumbs and milk in a large mixing bowl and let the bread crumbs soak up the liquid.
Once the onions are translucent, add them to the mixing bowl with the bread crumb mixture along with the pork, beef, allspice, nutmeg, salt, and pepper. Mix together until well combined.
Form the meatballs into 1 ½ inch size.
Add the second tablespoon of butter to the skillet and begin frying the meatballs in stages, flipping so that all the sides are browned. As the meatballs finish cooking, place them on a paper towel-lined plate.
Once all the meatballs are done, in the same skillet, add in 3 tablespoons of butter and melt. Whisk in the flour, and allow to cook for about 1-2 minutes.
Gradually whisk in the beef broth, and continue to whisk until gently thickened.
Add sour cream and worcestershire and whisk to combine. Add salt and pepper to taste.
Add meatballs back into the sauce and garnish with parsley, if desired.