Heat oil in a frying pan over medium-high heat.
Add onions and garlic. Cook, stirring occasionally, for about 3 minutes or until the onions are soft.
Add tomato paste, Dijon mustard and stock. Stir to combine. Season with salt and pepper.
Bring to a boil. Simmer for about 3 minutes.
Add meatballs. Simmer for about 3 minutes or until the sauce thickens slightly.
Stir in sour cream. Heat through.
Garnish with parsley.
Serve with your favorite side - pasta, rice, garlic bread, noodles, mashed potatoes.