Preheat oven to 425 degrees F.
Lightly oil a large shallow baking pan with 1 tablespoon olive oil.
Arrange peppers, cut sides up, in baking pan, and lightly oil the cut edges and stems.
In a bowl, toss the tomatoes, onion, basil, garlic, 2 tablespoons oil, red pepper flakes, mozzarella, and salt and pepper, to taste.
Divide mixture among peppers and roast in upper third of oven until peppers are tender, about 20 minutes.
Recipe courtesy of Gourmet Magazine