Prepare the meatballs: In a large bowl, combine the ground beef, beaten egg, almond flour, chopped parsley, salt, and pepper. Mix gently until all ingredients are combined.

Shape the meatballs: Roll the mixture into small meatballs, about 1 inch in diameter.

Cook the meatballs: Heat the butter or olive oil in a large skillet over medium heat. Once the pan is hot, add the meatballs and cook them for about 8-10 minutes, turning occasionally until browned on all sides and cooked through.

Make the mustard sauce: In a separate pan, combine the sour cream and Dijon mustard over low heat. Stir until the sauce is smooth and heated through. Be careful not to overheat or the sour cream may separate.

Coat the meatballs: Once the meatballs are cooked, transfer them to the pan with the mustard sauce. Toss the meatballs gently in the sauce to coat them evenly.

Serve: Serve the meatballs warm as a main dish or appetizer. Garnish with additional fresh parsley if desired.