Preheat oven to 350 degrees F.
Arrange half of rosemary sprigs in single layer in center of heavy baking sheet. Arrange sliced red onion on top of the rosemary. Place salmon, skin side down, on the 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. NOTE: Can be prepared 8 hours ahead, covered and refrigerated.
Bake, uncovered, 8 to 12 minutes or until a meat thermometer registers an internal temperature of 140 degrees F (salmon will be slightly opaque in thickest part). Remember the salmon continues to cook after it is removed from the heat source. Carefully remove salmon from pan and transfer onto individual serving plates. Serve with roasted onions and lemon slices.
Approximate cooking times for salmon:
1/4 to 1/3-inch - 3 to 4 minutes
1/2 to 3/4-inch - 4 to 6 minutes
1 to 1 1/2-inch - 8 to 12 minutes
This is the type of cooking and meat thermometer that I prefer and use in my cooking. I get many readers asking what cooking/meat thermometer that I prefer and use in my cooking and baking. I, personally, use the Thermapen Thermometer shown in the photo on the right. To learn more about this excellent thermometer and to also purchase one (if you desire), just click on the underlined: Thermapen Thermometer.
Makes 4 to 6 servings.