1. Thaw fish, if frozen. Place sliced onion in a single layer in the bottom of an ungreased 12x7.5x2 inch microwave-safe baking dish. Add water. Cover with vented clear plastic wrap. Microwave on 100 percent power (high) for 3 to 4 minutes, or until onion is crisp-tender.
2. Place fillets on onions slices and cook, covered, for to 5 minutes, or until the fish flakes easily when tested with a fork, giving the dish a half-turn after 3 minutes. Drain well.
3. Spoon picante sauce, tomato and olives over fish. Cook, uncovered, about 2 minutes more, or until heated through. Top with cheese. Cook for 1 minute, or until cheese is melted.