Mix the ground beef, breadcrumbs, egg, minced onion, nutmeg, salt, and pepper in a bowl until just combined. Shape the mixture into 16 to 20 small meatballs.

Melt 2 tablespoons butter in a large skillet over medium heat. Add the meatballs and cook, turning them gently, until browned on all sides and cooked through. Remove the meatballs to a plate and set aside.

Sprinkle the flour into the same skillet and stir for one minute to form a roux. Slowly whisk in the beef broth until smooth. Bring the mixture to a simmer and cook until it thickens slightly.

Stir in the heavy cream and return the meatballs to the skillet. Simmer for 5 minutes so the flavors blend and the gravy coats the meatballs evenly.

While the meatballs finish, place the cubed potatoes in a saucepan and cover with water. Bring to a boil and cook until the potatoes are fork-tender. Drain the water, then mash the potatoes with the milk and remaining 3 tablespoons butter until creamy. Season with salt to taste.

Spoon the mashed potatoes onto plates and top with the meatballs and gravy. Sprinkle with chopped parsley before serving.