Place the panko bread crumbs in a medium mixing bowl and set aside. In a skillet, melt 4 tablespoons of butter over medium heat. Add the diced onion, allspice, nutmeg, and black pepper. Cook until the onion becomes translucent. Add the garlic and cook for an additional 2 minutes. Pour in the milk and bring the mixture to a gentle simmer. Remove from heat and stir the mixture into the breadcrumbs. Set aside to cool.

In a separate mixing bowl, beat the eggs lightly. Add the ground meat and mix until combined. Once the breadcrumb mixture has cooled, add it to the meat mixture and combine thoroughly. Shape into golf ball–sized meatballs and place on a parchment-lined baking sheet. Refrigerate for 30 minutes. Preheat the oven to 375°F (190°C).

Bring a large pot of salted water to a boil. Add the shell pasta and cook until al dente, approximately 7–8 minutes. Drain and transfer the pasta to a 9×13-inch casserole dish. Set aside.

Wipe out the skillet used earlier and heat the olive oil over medium heat. Brown half of the meatballs on all sides, ensuring they are not overcrowded. Transfer browned meatballs to the casserole dish. Repeat with the remaining meatballs.

In the pasta pot, melt the remaining 8 tablespoons of butter over medium heat. Add the flour and stir continuously for approximately 5 minutes to cook the flour. Gradually add the beef broth, water, Worcestershire sauce, remaining black pepper, and heavy cream. Bring to a simmer and cook for 10–15 minutes, stirring occasionally, until the sauce thickens slightly.

Pour the sauce evenly over the meatballs and pasta. Transfer the casserole dish to the oven and bake for 30 minutes.

Remove from the oven and garnish with chopped parsley before serving.