Make a panade: In a bowl, combine the breadcrumbs and milk. Stir and let soak for 5 minutes until hydrated.
Prep aromatics: Grate the onion (or mince very finely). If desired, sauté the onion in a teaspoon of oil for 2–3 minutes to soften; cool slightly. Mince the garlic.
Mix meatballs: In a large bowl, add the ground beef, ground pork, egg, soaked breadcrumbs, onion, garlic, salt, pepper, allspice, and nutmeg. Gently mix just until combined—do not overwork.
Shape: Scoop into 1 to 1¼-inch balls (about 30–36 meatballs) and place on a sheet pan. Chill for 15 minutes to help them hold their shape.
Brown: Heat the olive oil in a large skillet over medium to medium-high heat. Brown the meatballs in batches, turning to color all sides, 5–7 minutes total. Transfer to a plate; they will finish cooking in the sauce. Leave the browned bits in the pan.
Make the roux: Reduce heat to medium. Melt the butter in the same skillet, then whisk in the flour. Cook, whisking, for 1–2 minutes until lightly golden and nutty.
Build the gravy: Slowly whisk in the warm beef broth, scraping up any fond. Simmer until slightly thickened, then stir in the heavy cream, Worcestershire, and Dijon (if using). Season with salt and pepper to taste.
Simmer to finish: Return the meatballs and any juices to the skillet. Spoon sauce over the top and simmer gently until the meatballs are cooked through (165°F/74°C), 8–10 minutes. Adjust seasoning as needed.
Serve: Sprinkle with parsley and serve with mashed potatoes or egg noodles. Offer lingonberry jam at the table for a traditional sweet-tart contrast.