In a large mixing bowl, combine ground beef, panko breadcrumbs, parsley, allspice, nutmeg, onion, garlic powder, salt, pepper, and egg. Mix gently until everything is just combined.
Roll the mixture into 12 large or 20 smaller meatballs and place them on a plate.
Heat olive oil and butter in a skillet over medium-high heat. Add the meatballs and cook for 7 to 8 minutes, turning occasionally.
Once browned, transfer them to a plate and cover to keep warm.
In the same skillet, melt the butter for the sauce. Whisk in flour and cook for 2 to 3 minutes until golden brown.
Gradually whisk in beef broth until smooth, then stir in heavy cream, Worcestershire sauce, and Dijon mustard. Simmer until it thickens.
Return meatballs to the skillet and simmer for 1 to 2 minutes until coated with sauce.
Serve the Swedish meatballs over egg noodles, mashed potatoes, or rice. Garnish with additional parsley if desired.