Melt 1 tbsp butter in the skillet over medium heat. Cook the grated onion for 3 to 4 minutes until soft and translucent, then scrape into a large mixing bowl and let cool. Wipe the skillet clean.
Add the panko and milk to the cooled onion, stir gently, and let sit for 5 minutes so the crumbs absorb the milk fully.
Add the ground beef, ground pork, beaten egg, allspice, nutmeg, salt, and pepper. Mix only until just combined and no dry streaks remain, then stop.
Use a small cookie scoop to portion the mixture and roll each portion into a smooth 1-inch ball, about 24 total. Chill for 15 to 30 minutes for firmer meatballs if time allows.
Melt the remaining 2 tbsp butter in the cleaned skillet over medium-high heat. Brown the meatballs in two batches, about 6 to 8 minutes per batch, then transfer to a plate.
Lower the heat to medium and melt 4 tbsp butter in the same skillet. Whisk in the flour and cook 1 to 2 minutes until light golden and nutty, scraping up the browned bits.
Pour in the beef broth slowly, whisking constantly to keep the sauce smooth. Stir in the Worcestershire sauce and Dijon mustard, then simmer 3 to 4 minutes to thicken.
Reduce the heat to low and stir in the heavy cream, keeping the sauce below a boil. Taste and adjust with salt and pepper.
Return the meatballs and any juices to the skillet, spoon sauce over the top, cover partially, and simmer 10 minutes until the meatballs reach 160°F (71°C). Garnish and serve hot.