Preheat the oven to 350 degrees F. Spray a 9-by-13-inch baking dish with nonstick cooking spray.
Heat the olive oil in a medium skillet over medium heat. Add the onion and green pepper, then sprinkle with salt and pepper and saute until tender, about 5 minutes. Add the garlic and Italian seasoning and cook for another 1 to 2 minutes. Remove from the heat and set aside.
In a large bowl, combine the meatballs, marinara sauce and cheese. Add the cooked onion and pepper mixture. Mix to combine, then transfer to the prepared baking dish. Top with the crushed croutons. Bake until the cheese is melted and bubbling, 30 to 35 minutes. (If the topping starts to get too brown, loosely cover with foil toward the end of the bake.)