In a large mixing bowl, mix all the ingredients for the meatballs together.
Heat a wide, heavy skillet over medium heat. Add 2 T olive oil to the pan and heat to shimmering.
Form 1½" meatballs and add them to pan. Be careful not to overcrowd the pan, work in batches if necessary. Brown the meatballs on all sides, turning as necessary.
Remove the meatballs from the pan and set aside. They don't need to be cooked through, they will finish cooking in the sauce.
Add the butter to the pan. When melted, whisk in the flour and cook for 1-2 minutes.
Slowly whisk in the beef stock, stirring until there are no lumps. Return the meatballs to the pan and cook for 10 minutes. Gently stir in the sour cream and mustard until combined.
Serve with egg noodles or mashed potatoes and sprinkle with fresh parsley.