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.
Meanwhile, stack the tortillas and seal in foil.
Cut a piece of heavy-duty foil the same size as salmon. Lift fish from marinade and place skin down on foil.
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.
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.
Pull off salmon skin and discard. Cut fish into 1-inch chunks and put in a bowl.
To assemble tacos, wrap chunks of salmon, salsa, cabbage, and feta in tortillas. Add salt to taste.