In a large bowl, combine ground beef, diced onion, egg, parmesan, parsley, garlic, salt, pepper, and nutmeg. Mix gently until just combined, do not overwork the meat.
Roll the mixture into evenly sized meatballs. Heat a large skillet over medium heat with a drizzle of oil. Cook the meatballs in batches, turning to brown all sides, until fully cooked. Remove and set aside.
In the same skillet, pour in the beef broth and bring to a gentle simmer. Whisk in the sour cream, Dijon mustard, and Worcestershire sauce until smooth and combined.
In a small bowl, mix the cornstarch with ¼ cup water to make a slurry. Slowly whisk it into the simmering gravy and cook for 2–3 minutes, until thickened.
Return the meatballs to the skillet and stir to coat in the sauce. Simmer for 5–7 minutes, until the flavors meld and the sauce thickens slightly.