In a skillet, melt 2 Tbsp butter, heating until melted and just starting to brown. Add onion to skillet, saute until onion is golden.
To the bread crumbs, add 2 Tbsp milk to moisten.
In a large bowl, combine beef, pork, egg. Add onion, mashed potatoes, and moistened breadcrumbs. Add spices (salt, pepper, allspice). Add remaining milk a little at a time; stop before the mixture gets gloppy. If the meat mixture gets too gooey to form nice, neat meatballs, add a more breadcrumbs.
Use a pair of spoons rinsed in water and shape the meat mixture into small round balls.
In a large skillet, heat remaining butter over medium heat. Add meatballs to skillet, being careful not to crowd the pan. Shake periodically so that the meatballs don't develop flat spots. Cook until meatballs are done through.