1. Pre-heat oven to 400 degrees F. In 13 by 19 inch baking dish, sprinkle potatoes with cumin and toss with oil to coat. Spread in even layer and roast until potatoes are browned, about 45 minutes.
2. Remove from oven. Increase oven temperature to 450 degrees. Use wide spatula to gently turn potato slices. Arrange fish on top of potatoes. Sprinkle with chili powder and scallions.
3. Return to oven and roast until fish is opaque in center, 8-10 minutes per inch of thickness.
4. Meanwhile, combine corn, green pepper, lime juice, cilantro, and jalapeno pepper.
5. With wide spatula, lift a portion of potatoes with fish on top onto serving plates. Spoon corn salad on top.