Add butter to the skillet. Once it melts, add the flour and whisk until it turns brown and smooth.
Slowly stir in beef broth and heavy cream, cook for about a minute.
Add garlic, thyme, Worcestershire sauce, and Dijon mustard. Slowly stir until the sauce comes together.
Add the sour cream and stir until the sauce is even throughout.
Bring sauce to a boil and simmer until it starts to thicken, 5-7 minutes.
Time for a taste test! Add salt and pepper as needed.
Add the meatballs back to the skillet and simmer them in the sauce for another 1-2 minutes.
Garnish with parsley and serve over egg noodles, mashed potatoes, or rice.