The secret to great, juicy keftedes is to make the mixture as loose as possible, but still dense enough that the little meatballs will hold their shape when fried. It is common practice in various parts of Greece to add grated tomato or milk, or both, to the mixture. The liquid adds the requisite moisture to the meat and changes its color so that the finished keftedes have a light, pinkish tint when broken apart. Frying the meatballs has its tricks, too. You actually don't need that much oil for this recipe, as the keftedes are pan-fried, not deep-fried. What you will need is to pay attention to the oil. It should not be so hot that the keftedes burn on the outside but remain undercooked within.
Combine the ground pork and onion in a mixing bowl. Add the tomato, mint, salt, and pepper, and knead well for 3 minutes to combine. Pour in the milk and continue kneading until the liquid is completely absorbed. If the mixture is so loose that the keftedes do not retain their shape, add a few tablespoonfuls of plain fine bread crumb. Cover the mixing bowl and refrigerate the mixture for at least 1 hour or up to 6 hours, so that the flavors meld.
To form, season the flour with salt and pepper, and spread out on a large plate. Have a second, clean plate ready. Take a tablespoonful at a time of the meat mixture and form into a small ball, about 1 1/2 inches in diameter. Roll in the flour and shake in the palm of your hand to remove the excess flour. Place the dredged meatball on the clean plate. Continue with the remaining mixture until all the meatballs are shaped.
To fry, heat about 1-inch of olive oil or other oil in a large, heavy skillet over medium heat. Using a tablespoon or small spatula, gently place as many meatballs as will fit in the pan without crowding. Fry them, turning once or twice so that all sides are browned. Remove, drain slightly on paper towels, and serve, either hot or at room temperature. The meatballs may be made several hours ahead and stored, covered, at room temperature.