Melt butter in a large skillet over medium heat. Cook and stir onion and salt in butter until onion begins to turn translucent, about 6 minutes.

Transfer onion mixture to a large bowl. Stir in milk, eggs, bread crumbs, black pepper, nutmeg, allspice, and cayenne.

Mix beef and pork into the bread crumb mixture. Cover with plastic wrap and refrigerate for 1 hour.

Melt butter for gravy in a large skillet over medium heat; whisk in flour and cook until golden brown, 4 to 5 minutes. Slowly whisk in beef broth.

Increase heat to medium-high, whisk cream into mixture and bring to a simmer. Stir in sugar and Worcestershire. Simmer until slightly thickened, 6 to 7 minutes. Season with salt and black pepper. Remove from heat and set aside.

Preheat the oven to 425 degrees F (220 degrees C). Line a baking sheet with aluminum foil and lightly coat with cooking spray.

Roll 2 to 3 tablespoons meatball mixture into a ball using wet hands; repeat with remaining meatball mixture. Place meatballs on the prepared baking sheet.

Bake in the preheated oven until browned, about 20 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).

Transfer meatballs to the gravy and cook over medium-low heat until warmed through, about 5 minutes.