Place the bread in a bowl, slightly broken up, and pour over the milk. Leave it to soak a few minutes while you prepare other ingredients.
Grate the red onion and mince or finely grate the garlic. Finley chop the mint and parsley.
Place the beef, pork, onion, garlic, mint, parsley, oregano, olive oil, vinegar, salt and pepper in a bowl. Squeeze out the milk from the bread with your hands (don't go overboard, just have it pretty well drained) and add the bread to the bowl with everything else.
Mix everything together so that it is well combined - it's best to do this at least partly with your hands as you need to break up the meats and bread so they mix properly. Cover the bowl and refrigerate for around 30 minutes to allow the flavors to mingle.
Prepare a small plate with flour on it for rolling and have a baking sheet or plate to put the meatballs on. Take roughly tablespoonfuls of the mixture and roll in your hands to form a ball. Roll it in the flour to coat on all sides then shake off excess flour and place on the baking sheet/plate. Repeat with the rest of the mixture (you should get around 24).
Warm a skillet/frying pan over a medium-high heat with a shallow layer of oil in it - you can use olive oil, canola or vegetable oil as you prefer or a mixture. You can use a large skillet to cook more at once or smaller if you prefer to not have to work as quick in turning them all etc.
Add some of the meatballs to the skillet with space between them so you don't overcrowd the skillet. Increase the heat a little if needed as the meatballs will reduce the heat and you want the oil to stay hot. Cook the meatballs around 2-3 minutes on each side and turn at least 2 or 3 times so that they are nicely browned and slightly crisp all over. A good sign that they are ready to turn is they should move without being stuck to the pan.
Once they are browned all over and cooked through, remove from the pan, draining with a slotted spoon or with spoon tilted to side, then drain the meatballs on kitchen paper to remove any excess oil. Repeat with the rest of the meatballs. Best served warm but also work at room temperature.