Cut peppers in half lengthwise. Remove and discard seeds and membranes.
Cook pepper cups in boiling water to cover 5 minutes. Drain and set aside.
Cook onion and garlic in butter in a skillet over medium heat 5 minutes or until tender, stirring constantly.
Combine onion mixture, breadcrumbs, and remaining 3 ingredients in a medium bowl. Spoon 3/4 cup onion-breadcrumb mixture into each pepper cup. Place pepper cups in an 11- x 7-inch baking dish. Bake, uncovered, at 400° for 20 minutes. Increase heat to broil. Broil 3 minutes or until lightly browned.