Preheat oven to 350°F. Line a rimmed baking sheet with parchment paper.
Heat neutral oil in a medium skillet over medium-high until shimmering. Add chopped onion and cook until translucent, about 3 minutes.
Transfer onion to a bowl, stir in fresh breadcrumbs, and let soak for 5 minutes. Stir in milk and egg yolk.
Add ground beef, ground pork, salt, allspice, nutmeg, and pepper. Mix gently until just combined—avoid overmixing.
Form mixture into ¾-inch meatballs, arrange on the prepared baking sheet, and chill for 15 minutes.
Heat more oil in the skillet over medium-high. Brown meatballs on all sides, about 2 minutes per side.
Return meatballs to baking sheet and bake until cooked through, 7–9 minutes.
Gravy
In the same skillet, melt butter over medium heat. Whisk in flour and cook until the roux turns golden, about 2 minutes.
Gradually whisk in chicken broth, bring to a simmer, then stir in heavy cream, Worcestershire sauce, and Dijon mustard. Simmer until thickened, about 3 minutes.
Season gravy with salt and pepper to taste.
Return meatballs to skillet, gently coat in gravy, and warm through.
Notes
Testing note: Pulse-chopping onion yields a silkier texture that prevents bits from catching on your teeth.For freezing, flash-freeze meatballs on a tray before bagging to preserve shape and ease portioning.