Add the panko breadcrumbs to a large bowl.
In a skillet over medium heat, melt 4 tablespoons of the butter.
Add the onion, 2 teaspoons of the salt, 1/2 teaspoon of the pepper, the allspice, and the nutmeg and cook, stirring often, about 5 minutes.
Add the minced garlic to the onion mixture and cook until it is fragrant, about 2 minutes.
Adjust the heat to medium-low and add the milk to the onion mixture, stirring everything together, and bring it to a simmer.
Pour the simmering onion mixture into the bowl of panko breadcrumbs and stir everything together.
Allow the breadcrumbs mixture to cool.
Add the eggs to a separate extra-large bowl and beat them.
Add the ground beef, the ground pork, and the cooled breadcrumbs mixture to the beaten eggs and mix everything together.
Form about 30 meatballs from the meat mixture.
Chill the meatballs in the fridge for 30 minutes.
In a large skillet over medium heat, heat the oil.
Add the meatballs to the hot oil in batches and cook until browned on all sides, about 15 minutes.
Transfer the meatballs to a dish.
Cook the shell pasta to al dente, according to the package directions.
Drain the pasta and pour it into a 9x13-inch baking dish.
Preheat the oven to 375 degrees F.
Place the meatballs into the baking dish and nestle them into the pasta.
In a large skillet over medium heat, melt the remaining butter.
Add the flour to the butter, stirring constantly, until the raw flour smell dissipates, about 2 minutes.
Stir the beef broth, the water, the Worcestershire sauce, the remaining salt, and the remaining pepper into the butter mixture and bring it to a simmer.
Reduce the heat to medium-low and add the parsley and the heavy cream to the broth mixture.
Cook the broth mixture, while stirring constantly, until it has thickened, about 10 minutes.
Carefully pour the sauce over the meatballs and the pasta.
Bake the meatball bake in the oven for 30 minutes.
Serve hot.