Cook meatballs according to package instructions, cover loosely with foil, and allow to rest for about 5 minutes.
Toss the ciabatta cubes with 1/4 cup of olive oil, salt, and pepper, place on a baking sheet, and cook at 375ºF until golden brown, about 7 minutes.
In a large mixing bowl, toss together to combine the remaining olive oil, balsamic, onions, and cherry tomatoes and season with salt and pepper.
Cut the meatballs into quarters, and add them to the bowl, tossing to combine.
Add the mint, arugula, and croutons, and toss one final time to coat.