If using frozen meatballs, heat them in the oven or microwave according to package instructions.
If using homemade meatballs, brown them in a large skillet over medium heat, then set aside.
In the same skillet, melt the butter over medium heat.
Add the diced onion and cook for 2–3 minutes.
Stir in the garlic and mushrooms and cook for another 3–4 minutes until softened.
Stir in the flour and cook for 1 minute.
Slowly add the beef broth, whisking to remove any lumps.
Stir in Worcestershire sauce, Dijon mustard, salt, and pepper.
Return the cooked meatballs to the skillet.
Let everything simmer for 5–7 minutes until the sauce thickens.
Remove the skillet from heat and stir in the sour cream.
Mix until smooth and creamy.
Spoon over cooked egg noodles and enjoy!