Combine beef and pork, breaking up lumps. Add onion, breadcrumbs, egg, garlic and mix slightly. Then add milk, salt, and pepper. Mix well.
Using a 1 Tablespoon cookie scoop (or a spoon), make 48-60 meatballs. Gently roll in your palms to create round balls. Lay the meatballs on a baking tray or plate, cover, and refrigerate for 2 (or more) hours*.
Preheat oven to 350˚F. Set aside a 2½-3 quart baking or casserole dish.
In a large non-stick skillet, heat about half of the oil on medium heat. Add about a third of the meatballs and brown on all sides. Add to the baking dish. Repeat with remaining oil and meatballs.
Cover the baking dish and bake the meatballs for 30 minutes.
Cream Sauce
In a medium skillet on low heat, melt butter. Whisk in flour and cook for 2 minutes. Add broths, whisking continuously. Add cream, soy sauce, and mustard, whisk well. Bring to a simmer and allow sauce to thicken, stirring often for about 5-10 minutes.
Serve meatballs and sauce with mashed potatoes or new potatoes. Garnish with parsley if desired.
Notes
*Meatball mixture is fairly wet, so refrigerating the meatballs helps them keep their shape when cooking.