Place bell peppers on an aluminum foil-lined baking sheet.
Broil 5 inches from heat (with electric oven door partially open) about 5 minutes or until bell peppers look blistered.
Place bell peppers in a heavy-duty zip-top plastic bag; seal and let stand 10 minutes to loosen skins. Peel, seed, and chop bell peppers.
Cut a thin slice from bottom of each onion, forming a base for onions to stand on. Remove onion centers, leaving 1/2-inch-thick shells. (Reserve centers for other uses.)
Stir together bell pepper, tomato, and next 6 ingredients; spoon evenly into onion shells. Place onions in a 9-inch square baking dish.
Bake, covered, at 400° for 50 minutes, and garnish, if desired.