Stir chicken, onion, garlic, fine salt, and 1/2 teaspoon black pepper together in a bowl until chicken is coated. Cover the bowl with plastic wrap and refrigerate, 4 hours to overnight.
Heat oil in a deep-fryer or large saucepan to 340 degrees F (171 degrees C).
Whisk cornstarch, flour, sugar, 1/2 teaspoon black pepper, and 1/4 teaspoon salt together in a large bowl. Gradually whisk ice water into flour in until mixture resembles a smooth pancake batter. Transfer chicken to batter and stir to coat chicken completely.
Working in batches, cook chicken in preheated oil for 4 minutes. Transfer cooked chicken to a cooling rack.
Increase oil temperature in the deep-fryer or large saucepan to 375 degrees F (190 degrees C).
Working in batches, cook chicken again in hot oil until golden brown and crispy on the outside, 3 to 4 minutes. Transfer to a wire rack to drain.