Heat grill on high or oven to 500 F
For the grill, make a foil tray, large enough for all the fish
Arrange about 1/2 the rosemary on the tray
Spread sliced red onion on top of rosemary.
Drizzle some olive oil around.
Place salmon, skin down, on top of onions and add salt and pepper.
Arrange more rosemary on top of fish
Top with the lemon slices and more olive oil.
Grill for about 15 minutes, until just cooked through.