Combine chicken mince, crushed garlic, one and a half tablespoons Thai chilli sauce, lemon rind and two teaspoons lemon juice in a bowl.
Transfer into a mixer jar and grind till smooth. Divide into four portions.
Heat two tablespoons oil in a non stick grill pan. With wet hands take each portion of the chicken mixture and shape into a patty. Place it in the pan and grill, turning sides once, till both the sides are evenly cooked and golden. Slice the halved onion
Heat the remaining oil in a non stick pan, add the onion and sauté till lightly coloured. Add the remaining Thai chilli sauce and the remaining lemon juice and sauté for half a second.
Transfer into a bowl. Halve the burger buns horizontally and warm them on a heated pan till lightly crisp. Place them on the work top. Place lettuce leaves over one half. Dribble a little of the onion chutney over it and place the chicken patty on it.
Cover with the second half and serve immediately with the remaining onion chutney.