Heat deep fryer to 365 F.
Whisk together flour, cornstarch, baking powder and salt and pepper.
Add ginger beer (see notes) and stir just until combined. If batter is over mixed, it will be tough.
Dredge halibut in cornstarch then dip in batter.
Allow any excess batter to drain away, then slowly, carefully, dip the fish into the hot oil.
Fry until golden and crispy and fish is cooked through, approximately 8 to 10 minutes. The oil temperature will drop a bit, 365 degrees is ideal.