1. Preheat broiler. Quarter peppers lenghwise; remove seeds and membranes. Place bell pepper, skin side up, on an aluminum foil-lined baking sheet. Broil 5 inches from heat 8 to 10 minutes or until bell pepper looks blistered. Place bell pepper in a large zip-top plastic freezer bag; seal bag. Let stand 10 minutes to loosen skins. Remove bell pepper from bag, reserving bag. Peel bell pepper; cut into 1/2-inch-thick slices.
2. Place oil and next 5 ingredients in reserved bag. Seal bag; shake to blend. Add roasted bell pepper, onion, and next 3 ingredients; seal bag, and shake to coat vegetables. Chill up to 24 hours, turning bag occasionally. Remove and discard rosemary sprig before serving.