Preheat grill to medium or medium high while preparing the salmon.
Lay two layers of 18” long heavy duty aluminum foil on top of one another on the counter.
Lay two slices of lemon onto the middle of the aluminum foil.
Cut one slice of onion and separate into rings. Lay these on top of the lemon slices.
Sprinkle both sides of the salmon with salt and pepper and lay the filet on top of the lemon and onion slices.
Drizzle the olive oil over the surface of the salmon and sprinkle with garlic and cayenne.
Lay sprig of Rosemary on the top of the Salmon.
Fold the aluminum foil from each end and roll together.
Pull the side of the aluminum foil to the top and seal together by rolling and pinching.
Repeat for each piece of salmon you will serve.
When the grill has come to temperature (make sure the grill grates are very clean so the aluminum foil will not stick), set the packet onto the grill grates and replace the top on the grill. Allow to set for 9 minutes.
Remove the packet from the grill to a platter and allow to sit, closed, for 1-2 minutes