Slice lemon and place in a bowl.
Boil ½ cup water in a non-stick pan, add sugar and stir continuously till sugar dissolves.
Put 1 cup refined flour, salt, green chilli sauce, 2 tsps oil and egg white in a bowl and mix well. Add 3 tbsps water and whisk well to make a thick and smooth batter.
Spread sufficient refined flour on a plate.
Dip the fish fillets in the batter and coat with refined flour.
Heat an air fryer and brush the basket with oil. Place the prepared fish fillets in it, fit the basket to the fryer and cook at 180º C for 15-20 minutes.
Add salt to the pan with the syrup and mix well. Add cornflour slurry and mix again. Add red chilli sauce and mix well. Add lemon slices, lemon juice and mix well. Cook till the lemon sauce thickens.
Remove the fish from air fryer basket, brush with some oil and place in the air fryer basket again. Continue to cook for 5 minutes more.
Roughly tear lettuce leaves and make a bed on a serving platter. Place air fried fish over the lettuce, pour lemon sauce over them and serve immediately.