Season fish fillet with salt and pepper. Coat with cornstarch. Heat a small amount of oil in a pan. Place the fish in the hot oil Lightly brown the fish, adjusting heat as needed. Turn and finish cooking.
Heat a few tablespoons of oil in a separate pan. Add mushrooms and garlic. When the mushrooms release iquid, allow a little of the liquid to dry up. Add the scallions and toss quickly. Add sherry and then cashews.