For the burgers, using a hand blender, roughly chop the onion, ginger, garlic, coriander and green chillies by pulsing, or chop them all by hand. Place into a bowl with the lamb mince, add the cumin, garam masala, salt, egg and breadcrumbs and mix well.
Shape into six burgers and chill in the fridge for 20 minutes, or until you are ready to cook them. Take them out of the fridge 30 minutes before you start cooking.
Meanwhile, for the herbed yoghurt, mix together all the ingredients for the herbed yoghurt in a bowl and season, to taste, with salt and freshly ground black pepper.
Preheat the grill to high. Place the burgers on a well-oiled baking sheet and cook for about 4-5 minutes on each side. They can also be cooked in an oiled non-stick pan for 8-10 minutes, turning halfway.
While the burgers are cooking, heat the buns in the oven for the last 3-4 minutes of the cooking time.
To serve, split the buns in two. Place a burger onto one half of each bun on a bed of lettuce. Top with the tomatoes and onion rings and a spoonful of the herbed yoghurt. Finish with the other half of each bun.