Pre-heat the oven to 390F. Line a baking sheet with foil for easy clean-up.
Cut at least 5 deep slits on each side of the fish. Do not go through the spine or to the other half of the fish.
Cut the rosemary into sections that will fit inside each of the slits you cut. Place one piece of rosemary inside the each of the slits cut in the fish. Place 2-3 slices of garlic in each slit. Season each cut with salt.
Slice 1 lemon into 5-6 slices. Place the slices of lemon inside the fish.
Drizzle olive oil and generously season with salt on both sides of the fish.
Cook 13-15 minutes. It is ready when you pull a fin and it comes out easily.
Skin (or not) and serve! Once you filet the fish on one side, the spine comes out easily and the other side is ready to eat!
Video
Notes
You can eat the skin as long as the scales are fully removed.