Mix the ground beef, onions, bread crumbs, buttermilk, egg, parsley, salt, pepper, nutmeg, allspice and garlic powder together until just combined. DO NOT OVER MIX.
Shape into about 25-30 meatballs. (I use a 1 1/2" cookie scoop.)
Preheat a large skillet over medium-high heat. Add olive oil and 1 tablespoon butter. Add the meatballs and cook turning continuously until brown on each side and cooked through. Transfer to a plate and cover loosely with foil.
Add 3 tablespoons butter to skillet, once melted add the 2 tablespoons of flour, whisk and cook for 1-2 minutes.
Slowly whisk in beef broth.
Lower the heat and add the sour cream, whisk until incorporated.
Add Worcestershire sauce and mustard and bring to a simmer until sauce starts to thicken. Salt and pepper to taste.
Add the meatballs back to the skillet and simmer for another 1-2 minutes.
Serve over egg noodles or rice.