Heat oil in a non-stick pan, add ground green chillies and sauté lightly.
Add peanuts, walnuts, cashewnuts and sauté for 1-2 minutes.
Add water chestnuts, mix well and sauté for 5-7 minutes. Transfer into a bowl and set aside to cool for 5 minutes.
Add potatoes, cornflour, lemon juice, chaat masala and salt and mix well.
Dust your palms with cornflour. Divide the mixture into equal sized balls and shape them into tikkis.
Grease the basket of the air fryer with a little oil. Place the tikkis in the basket and fit it to the fryer and cook 180°C for 9-10 minutes.
Place a lemon wedge, thin strips of red, yellow and green capsicums on a serving plate. Place the tikkis on the plate and serve hot.