Prepare covered grill. Spread a thin layer of mayo over the bottom of a large aluminum foil roasting pan. Place salmon on top of mayo. Season the cavity with salt and pepper and tuck in onion and half of the lemon slices. Spread mayo on top of salmon and cover with more lemon slices. Place foil pan on grill rack, cover grill, and grill over hot coals until fisk flakes when tested with a fork, 8-10 minutes per inch of thickness of fish. (The salmon can also be baked in a preheted 400* oven, allowing 8-10 minutes per inch of thickness of fish). When grilling, a smokier flavor can be obtained by the addition of a few water-soaked alder wood chips to the coals the last half hour of cooking.