Rinse salmon fillet under cold running water and pat dry. Cut a piece of foil long and wide enough to encase the salmon when folded over the center and folded up on the ends. Position the foil on a large baking sheet and place the salmon in the center of the foil, skin side down. Sprinkle the salmon with the salt, pepper, and rosemary. Place the lemon slices over the fish fillet along with the sliced onion and garlic slivers. Drizzle the fillet evenly with the olive oil and fold the edges of the foil up over the salmon and crimp to seal. Fold the edges together on both ends to create an airtight package.
Bake the salmon for 15 minutes, or until just cooked through but still moist. Serve immediately, using a large metal spatula to portion fish. Serve some of the accumulated juices spooned over the fish.