In a large bowl, combine the ground beef, panko breadcrumbs, chopped onion, egg, allspice, nutmeg, and black pepper. Mix gently until just combined. Be careful not to overmix.

Shape the mixture into small, bite-sized meatballs about 1 inch in diameter.

In a large pot or Dutch oven, melt 2 tablespoons of butter over medium heat. Add the diced onion, carrots, and celery. Cook for 5-7 minutes, stirring occasionally.

While the vegetables are cooking, heat the olive oil or butter in a separate skillet and brown the meatballs in batches for about 2-3 minutes on all sides.

Sprinkle the flour over the softened vegetables and stir to coat. Cook for one minute, stirring constantly.

Gradually whisk in the beef broth until smooth, scraping up any browned bits from the bottom. Stir in the cream of mushroom soup, heavy cream, and Worcestershire sauce.

Bring the soup to a simmer, then add the browned meatballs. Reduce the heat to low, cover, and let the soup simmer for 15-20 minutes.

Season with salt and pepper to taste. Ladle into bowls and garnish with fresh dill or parsley.