In deep fat fryer, heat 2 inches of oil to 375°. In medium mixing bowl, combine cornstarch, egg yolks, beer, soy sauce, ginger, mustard and 1/4 teaspoon salt. Set aside.
In another medium mixing bowl, beat egg whites at high speed of electric mixer until stiff but not dry. Fold into beer batter.
In shallow dish, combine flour, remaining 1/4 teaspoon salt and the pepper. Dip fish first in batter and then dredge in flour mixture to coat. In hot oil, fry fish, a few pieces at a time, for 3 to 4 minutes, or until golden brown, turning over once. Drain on paper towel-lined plate.