Finely chop the onion and sweat it in 1 tbsp butter over medium heat until soft and translucent; cool slightly.
Combine breadcrumbs and milk in a bowl and let soak for 5 minutes until softened.
In a large bowl, add ground beef, ground pork, soaked breadcrumbs, cooled onion, egg, salt, pepper, allspice, and nutmeg.
Mix gently with your hands or a spoon until just combined; avoid overworking to keep meatballs tender.
Wet your hands and shape mixture into small walnut-sized meatballs (about 1 to 1¼ inches); place on a tray.
Heat 1 tbsp oil and 2 tbsp butter in a large skillet over medium-high heat.
Add meatballs in batches, not crowding the pan; brown on all sides, about 6–8 minutes total per batch, transferring browned balls to a plate.
Reduce heat to medium, add 2 tbsp butter to the same skillet, stir in 2 tbsp flour and cook 1–2 minutes to form a roux, scraping browned bits.
Gradually whisk in stock until smooth, simmer until slightly thickened, then stir in cream, Dijon mustard, and Worcestershire; season to taste.
Return meatballs and any collected juices to the sauce, simmer gently 6–8 minutes until cooked through and sauce coats meatballs.
Finish with a squeeze of lemon if desired and sprinkle chopped parsley before serving.