1. Preheat broiler. Cut sides off peppers and arrange in a single layer on a large sheet pan. Brush peppers with oil and season with salt and pepper. Broil 5 inches from heat, turning, until charred in spots and flesh is tender, about 5 minutes. Immediately drizzle vinegar over peppers and broil 30 seconds more.
2. Transfer peppers with any juices to a bowl. Add rosemary and salt and pepper to taste; toss to combine. Makes 12 servings.