1. Place salmon in a heavy-duty plastic food bag. Add onion, lime juice, garlic, pepper, and oil. Seal bag. Turn fish often for 30 minutes.
2. Meanwhile, stack the tortillas and seal in foil.
3. Cut a piece of heavy-duty foil the same size as salmon. Lift fish from marinade and place skin down on foil.
4. Lay foil with fish on a barbecue grill over a solid bed of medium coals or gas grill on medium (you can hold your hand at grill level only 4 to 5 seconds). Evenly distribute onion from marinade over fish. Close lid on grill; open vents for charcoal.
5. After 5 minutes, place tortilla packet next to salmon. Turn tortillas every few minutes. Cook until salmon is no longer translucent but is still moist-looking in center (cut to test) and tortillas are hot in center, about 8 minutes more.
6. Pull off salmon skin and discard. Cut fish into 1-inch chunks and put in a bowl.
7. To assemble tacos, wrap chunks of salmon, salsa, cabbage, and feta in tortillas. Add salt to taste.