Preheat the oven to 425°. Place a bowl of warm water on a work surface. Arrange a rack on a rimmed baking sheet. Divide the sausage into 4 equal portions. Form 3 balls from each portion, nestling 1 mozzarella cube into the center of each meatball and sealing the meat around it. Coat the meatballs with a drizzle of EVOO, place on the rack on the baking sheet and roast until firm, about 18 minutes.
While the meatballs are cooking, in a large skillet, heat 1 tablespoon EVOO, 1 turn of the pan, over medium heat. Add the onion and garlic and cook until tender, about 5 minutes. Stir in the crushed tomatoes, sun-dried tomatoes and olives; season with salt and pepper and simmer until the sauce thickens, about 7 minutes.
Remove the meatballs from the oven, turn off the heat, add the rolls and bake until crisp, 3 minutes. Split the rolls; top each with 3 meatballs and some sauce, basil and pecorino.