Heat the oven to 375°F Cut the tops off the peppers and reserve. Remove and discard the seeds. Place the peppers, cut-side up, into a 3-quart shallow baking dish.
Heat the oil in a 10-inch skillet over medium-high heat. Add the onions and garlic and cook until tender. Place the onion mixture into a medium bowl. Stir in the chicken, sauce, cheese and black pepper. Spoon the chicken mixture into the pepper halves. Top with the reserved pepper tops, if desired.
Bake for 40 minutes or until the peppers are tender.
TIP: Cut a thin slice from the bottoms of the peppers to keep them from rolling in the dish.