Blend the meat and the seasonings (salt, sage, cumin, and allspice) together. When you measure out your milk, crack your egg into that same measuring cup and whisk the egg and milk together. Add the rice, milk, and egg, and blend well. The mixture will look quite wet at first, but it will come together. Remove the meatballs with tongs or a spoon and set aside.
Form the meat into balls, ~2" wide (bigger meatballs are more prone to falling apart as you brown them), and quickly brown over medium heat in the fat of your choice (you don't need to cook them through, just lightly brown the outside). The meatballs may be delicate because the liquid content is high, so handle them carefully.
Put the tomato, water, onion, rosemary, and remaining 1 teaspoon salt into a large pan, saucier, or pot. You'll need enough space for all the meatballs to fit in it as well. Bring that to a boil and then nestle the meatballs into the tomatoes. Cover and simmer on low for 1.5 hours.