In a large bowl, combine ground beef, ground pork, breadcrumbs, salt, pepper, chopped parsley, Worcestershire, and onion. Mix well. Beat the egg with the milk and add to the meat mixture, combining thoroughly.

In a large skillet, melt the butter over medium heat and brown the meatballs on all sides.

Remove the meatballs from the skillet. In the same skillet, add flour to the remaining butter and stir to make a roux. Gradually add beef broth, stirring continuously until the sauce thickens.

Return the meatballs to the skillet, add capers, and simmer for 25 minutes. Add more broth while cooking if the sauce becomes thick.

Just before serving, stir in the sour cream. Avoid boiling after adding sour cream to prevent curdling. Sprinkle with fresh chopped parsley for garnish.