Preheat the oven to 400°F with two racks close to the center. Line two rimmed baking sheets with parchment.
Place the beef and pork into a large mixing bowl.
Using a fork or your hands, gently but thoroughly break up the meat and stir to combine. The key with mixing is to avoid compressing the meat and think instead of lifting as much as possible.
Add the grated cheese, salt, pepper, bread crumbs, eggs, milk, garlic, parsley, basil, red pepper flakes, and nutmeg.
Use a fork or your hands to mix gently but thoroughly until the mixture is homogenous but still light.
Use a 3-tablespoon ice cream scoop to divide the mixture into 22 meatballs. Place half on each of the baking sheets, with plenty of room in between for the air to circulate in the oven.
Use your hands to finish shaping the scoops into balls, again without compressing too much.
Bake for 25 minutes, swapping the position of the trays halfway through cooking.
In a large frying pan, heat the tomato sauce until it simmer. Add the meatballs and simmer for a minute or two to meld the flavors. Meatballs are ready to serve.