Combine the yogurt, sriracha, and lime juice. Set aside. Heat the oil in a large cast-iron skillet over high heat. Rub the fish fillets on both sides with plenty of blackening seasoning. When the oil in the pan is smoking, add the fish and cook, undisturbed, for 3 minutes, until a dark crust forms. Flip the fillets and cook for an additional 2 to 3 minutes, until the fish flakes with gentle pressure from your finger. While the fish is cooking, toast the buns (cut side up) under the broiler. Divide the avocado and cabbage among the buns. Top with the hot fish, yogurt sauce, and onions.