Thinly slice a quarter of the onion and mix with the tomatoes and half the coriander. Season and set aside.
Coarsely grate the remaining onion and mix well with the turkey mince, sweet chilli sauce, soy sauce, remaining coriander and plenty of black pepper. Form into 4 burgers, which are slightly larger than the burger buns. (The mixture will be quite wet, but, when cooked, will give you moist, juicy burgers.).
Preheat the grill to hot. Arrange the burgers on a foil-lined grill pan and grill for 6 minutes on each side.
Meanwhile, halve the cucumber lengthways, scoop out the seeds with a teaspoon and discard. Dice and mix with the yogurt, mint, seasoning and Tabasco.
Grill the burger buns until toasted. Arrange a burger on each toasted bun base, top with a spoonful of tzatziki, then tomato salad and finally the burger tops to serve.