In a large mixing bowl, combine ground beef with breadcrumbs, egg, milk, chopped onion, parsley, garlic powder, allspice, salt, and pepper. Mix well to form a uniform consistency.
Take 2 spoonfuls of the beef mixture at a time and roll into meatballs. Set aside and lightly coat them with some oil to prevent sticking, if needed.
In a large skillet, heat butter over medium-high heat for 2 minutes until melted butter sizzles. Add meatballs and cook until browned on all sides, about 7-8 minutes. Stir gently to prevent disintegration. Set aside on a plate.
In the same skillet, add olive oil and heat over medium-high heat for one minute. Stir in the flour, allowing it to bubble and thicken. Whisk in beef broth and heavy cream. Keep stirring until the mixture is uniform and thickened, about 1 minute. Stir in Dijon mustard and Worcestershire sauce. Season with salt and pepper.
Add the meatballs back into the skillet and toss to coat. Cover with a lid and cook over medium-heat until meatballs are cooked through, about 5 more minutes. Continue stirring the cream sauce until it reaches your desired consistency, about 2 minutes.