Stir the breadcrumbs and milk together in a large bowl and let them stand for 5 minutes.
Add the beef, pork, egg, onion, salt, allspice, nutmeg, and pepper. Mix gently until just combined, then shape into 1 1/2-inch balls.
Melt the butter in a large skillet over medium heat. Brown the meatballs in batches, turning often, until browned and cooked through. Transfer to a plate.
Make the Cream Sauce:
Melt the remaining butter in the skillet. Whisk in the flour and cook for 1 minute.
Gradually whisk in the broth. Add the cream, soy sauce, and mustard and simmer until the sauce lightly coats a spoon.
Return the meatballs to the skillet and simmer gently for 5 minutes, turning them in the sauce.
Serve:
Spoon the meatballs and sauce over mashed potatoes.
Serve with lingonberry jam and pickled cucumber.
Notes
Do not overmix the meat mixture or the meatballs can become dense.
Lingonberry jam provides the traditional sweet-tart contrast to the savory gravy.
A splash of broth loosens the sauce when reheating.