Soak the roll in water, then squeeze it out well. In a bowl, mix the minced meat with parsley, finely chopped mint, finely chopped onion, cumin, egg, Parmesan, and squeezed roll. Season well with salt and pepper. Cover and refrigerate for 30 minutes.
Shape the meat mixture into small meatballs using slightly moistened hands. Roll the meatballs in flour. Heat olive oil in a frying pan over medium-high heat. Fry the meatballs for 8-10 minutes, turning occasionally, until they reach an internal temperature of 71°C (160°F). Remove with a slotted spoon and drain on kitchen paper.
Thinly slice the second onion. Heat a little olive oil in a frying pan over medium heat. Fry the onion slices for 3-4 minutes until softened. Add the cooked meatballs back to the pan. Pour in the balsamic vinegar and swirl for 1 minute. Serve immediately, garnished with the roughly chopped fresh mint.