Crispy Chinese Lemon Chicken - old school style!

Bahan-bahan (4 porsi)

• 4 x 125g/3oz chicken thigh fillets (, skinless and boneless (or 2 breasts - Note 1))
• 1/2 tsp cooking salt / kosher salt
• 1/4 cup cornflour / cornstarch
• 1 - 1 1/2 litres (4 - 6 cups) vegetable or canola oil (, for frying - 4cm / 1.6" depth (re-usable, Note 2))
• 1/3 cup cornflour / cornstarch
• 1/3 cup + 1 tbsp plain flour ((all-purpose flour))
• 1/2 tsp cooking salt / kosher salt
• 1/4 tsp baking powder
• 1/2 cup COLD soda water (, not sparking mineral water (Note 3))
• 6 tsp cornflour/cornstarch
• 1/2 cup chicken stock/broth (, low sodium)
• 1/2 cup lemon juice (, fresh)
• 1/3 cup white sugar
• 1/2 tsp garlic (, finely grated)
• 1/2 tsp ginger (, finely grated)
• 1 tbsp Chinese cooking wine ((can omit for alcohol-free))
• Lemon slices (, to serve (optional))
• 1 g reen onion (, finely sliced on the diagonal, to serve (optional))