Cut peppers in half lengthwise; discard seeds and membranes. Place pepper halves, skin side up, on a large aluminum foil-lined baking sheet; flatten peppers with palm of hand. Add onion to baking sheet. Broil 5 1/2" from heat 20 to 25 minutes or until peppers are blackened. Place peppers in a zip-top plastic bag; seal, and let stand 15 minutes. Peel skin from peppers; cut pepper into strips.
Whisk together vinegar and next 4 ingredients in a medium bowl; stir in rosemary. Add pepper strips and onion to bowl; toss well.