Preparation Preheat oven to 500°F. Arrange half of rosemary sprigs in single layer in center of heavy baking sheet. Arrange sliced red onion atop rosemary. Place salmon, skin side down, atop red onion. Sprinkle with salt and pepper. Cover salmon with remaining rosemary sprigs. Arrange lemon slices over rosemary. Drizzle olive oil over. Sprinkle lemon slices with salt. (Can be prepared 8 hours ahead. Cover and refrigerate.) Roast salmon until just cooked through, about 20 minutes. Transfer salmon to plates. Serve with roasted onions and lemon slices.
Preparation Preheat oven to 500°F. Arrange half of rosemary sprigs in single layer in center of heavy baking sheet. Arrange sliced red onion atop rosemary. Place salmon, skin side down, atop red onion. Sprinkle with salt and pepper. Cover salmon with remaining rosemary sprigs. Arrange lemon slices over rosemary. Drizzle olive oil over. Sprinkle lemon slices with salt. (Can be prepared 8 hours ahead. Cover and refrigerate.) Roast salmon until just cooked through, about 20 minutes. Transfer salmon to plates. Serve with roasted onions and lemon slices.