Soak the stale bread in the milk and set aside for five minutes, until the bread has absorbed the milk and softened.
While the bread is soaking, make the sauce.
Pour the 1 1/2 bottles of strained tomatoes into a 10 inch or larger skillet. Fill the empty bottle with water 3/4 of the way to the top and add the water to the skillet.
Add in the onion, carrot, and butter.
Turn the heat to medium and cover to simmer, while you prep the meatballs.
Add the ground beef to a large bowl along with the salt, oregano, garlic, black pepper, parmesan cheese, egg, mint leaves and basil leaves.
Squeeze as much milk out of the soaked bread as possible. Add to the meat mixture.
Using clean hands, mix the meat mixture together until well combined and there are no clumps of bread throughout.
Form into about 15 golf ball sized balls. Press into each ball gently, and place one bocconcini inside. Cover the cheese well and gently roll the ball between two hands.
Gently drop the meatballs directly into the simmering sauce, one at a time.
Gently turn each meatball over to coat with the sauce. Cover and simmer on medium low for 45 minutes, or until the carrots have softened.
Remove the carrots and onions from the sauce and enjoy!