Soak the breadcrumbs. Stir the breadcrumbs and milk together in a small bowl and let them sit for about 5 minutes, until the breadcrumbs have softened and absorbed most of the milk.
Cook the onion and garlic. Melt 1 teaspoon butter in a small skillet over medium heat. Add the finely chopped onion and cook for about 3–4 minutes, until softened. Stir in the garlic and cook for another 30 seconds. Remove from the heat and let the mixture cool slightly.
Mix the meatballs. Add the ground beef, ground pork, soaked breadcrumbs, egg, cooked onion and garlic, salt, black pepper, allspice, and nutmeg to a large bowl. Gently mix just until everything is evenly combined. Try not to overwork the meat mixture.
Shape the meatballs. Roll the mixture into meatballs about 3–4 cm in diameter, keeping them fairly similar in size so they cook evenly.
Brown the meatballs. Heat 1 tablespoon butter and 1 tablespoon oil in a large skillet over medium-high heat. Add the meatballs in batches and cook for about 5–7 minutes, turning them as they brown. They should develop a nice golden-brown crust on the outside. The centers may still be slightly undercooked at this point, and that’s okay because the meatballs will finish cooking in the gravy. Transfer them to a plate and keep the browned bits in the skillet.
Start the gravy. Reduce the heat to medium and melt 3 tablespoons butter in the same skillet. Add the flour and stir continuously for about 1 minute.
Add the beef broth. Slowly pour in the beef broth, adding it gradually while stirring or whisking so the gravy stays smooth. Scrape up the browned bits from the bottom of the skillet as you go.
Make the gravy creamy. Stir in the heavy cream, Worcestershire sauce, Dijon mustard, salt, and black pepper. Bring the gravy to a gentle simmer and let it cook briefly until it begins to thicken.
Finish the meatballs in the gravy. Return the browned meatballs to the skillet along with any juices collected on the plate. Let them simmer gently in the gravy for about 8–10 minutes, turning them once or twice, until cooked through and the gravy is smooth and creamy.
Finish and serve. Taste the gravy and add a little more salt or pepper if needed. If the sauce becomes thicker than you like, stir in a small splash of beef broth until it reaches the consistency you want. Serve warm with plenty of gravy.