In a large bowl, combine the ground beef, panko breadcrumbs, chopped onion, egg, allspice, nutmeg, and black pepper. Mix gently until just combined - do not overmix as this makes the meatballs tough.

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

In a large pot or Dutch oven, melt the 2 tablespoons of butter over medium heat. Add the diced onion, carrots, and celery and cook until softened, about 5-7 minutes.

While the vegetables are cooking, heat olive oil in a separate skillet over medium-high heat. Brown the meatballs in batches on all sides (this step is optional but adds great flavor). Remove the meatballs and set aside.

Sprinkle the flour over the softened vegetables in the pot and stir to coat. Cook for one minute to remove the raw flour taste.

Gradually whisk in the beef broth until smooth. Stir in the cream of mushroom soup, heavy cream, and Worcestershire sauce.

Bring the soup to a simmer, then add the browned meatballs to the pot.

Reduce the heat to low, cover, and let the soup simmer for 15-20 minutes, or until the meatballs are cooked through and the flavors have melded together.

Season with salt and pepper to taste.

Ladle the soup into bowls and garnish with fresh dill or parsley before serving.