Cut fish into uniform 1x2 inch pieces. Dry fish pieces. Put cornstarch on a plate. Combine egg white, 1 tablespoon of the sherry, ginger and pepper in a small bowl. Dip fish first in cornstarch to coat thoroughly, then in egg white mixture. Heat oil in a wok or large skillet. When oil is very hot, add coated fish pieces and fry them quickly until they are golden brown on both sides, kee ing them moving and turning them as they cook. Remove fish to a warm serving dish. Add mushrooms and scallions to wok or skillet and stir-fry a minute two. Add remining sherry and soy sauce, stir and pour contents of pan over fish. Serve at once.