Soak eight or 12 8-inch wooden skewers in water for 10 minutes.
In a small bowl or cup, mix cumin with onion purée and a little lemon juice to make a paste.
Add more juice as needed.
Stir in oil.
Rinse fish and pat dry with paper towels.
If using whole fish, cut a few deep slashes in the sides to allow the seasonings in and to speed cooking.
Rub a little of the seasoning mixture on the whole fish, inside and out, or on both fillet sides.
Thread fish on skewers (use an in-and-out"basting stitch") and grill over hot coals for about five minutes on each side or until fish flakes easily when tested with a fork.