Combine chicken, grated onion, garlic, salt, and pepper in a bowl. Cover and refrigerate for 4 hours or overnight.
Heat oil to 340°F (171°C) in a deep fryer or heavy pot.
Whisk cornstarch, flour, sugar, pepper, and salt. Gradually add cold water to form a smooth batter.
Dip marinated chicken into batter, coating fully.
Fry chicken in batches for 4 minutes. Transfer to a wire rack.
Increase oil temperature to 375°F (190°C).
Fry chicken again for 3–4 minutes until golden and crispy.
Drain and serve hot.
Notes / Tips:
For substitute self-rising flour, use 1/2 cup all-purpose flour, 3/4 teaspoon baking powder, and 1/4 teaspoon salt.
Serve plain or with Korean fried chicken sauce.