Cut a piece of heavy-duty aluminum foil, 12"x12". Grease lightly. Place a portion of fish, skin side down, on foil and generously pour Sophia's Tahini Dressing over the fish. Top with sliced red onion. Bring the foil up over the fish and close all edges with tight double folds. Place packages on a grill about 5" from moderately hot coals. Cook for 10 to 20 minutes or until fish flakes easily when tested with a fork. For an extra finish, open the foil and cook until Tahini and onions turn golden brown on top.