Preheat the oven to 450 degrees F.
Add the tomatoes to a shallow 2-quart casserole dish in a single layer.
Toss the tomatoes with the oil, and salt and pepper, to taste. Make sure the tomatoes are cut side up after tossing. Roast until the tomatoes are extremely tender, about 35 to 40 minutes. Remove them from the oven and keep warm.
In a large salad bowl, toss the arugula, mozzarella balls, basil and the warm tomatoes along with any juices from the dish. Drizzle with some good balsamic and season with salt and pepper, to taste.