1. Mix together all the ingredients for the burgers (except the buns and sliced tomato), season with 1¼–1½ tsp salt and leave to rest for 30 minutes. You can either cook the chillies on the barbecue later, or do them in a frying or griddle pan in advance. Wherever you cook them, place them directly on the hot surface and cook, turning once the underside has charred and blistered. Once they are done, wrap in cling film and set aside
2. Heat the oil for the onions in a frying pan and, once hot, add the panch phoran. Cook for 20–30 seconds, or until the seeds are popping and colouring. Add the onions and a good pinch of salt and cook over a high heat until they have coloured and are well browned on the edges, 5–7 minutes. Adjust the seasoning and set aside. (You can reheat these on the barbecue later in a flameproof pan, or keep them warm in a low oven.)
Mix together the yogurt, mayonnaise and coriander for the topping, adding a good grinding of black pepper and salt lightly. Once the chillies are cool, peel off their skins, slit lengthways and deseed; discard the seeds. Then chop the flesh up and add to the yogurt. Set aside.
When you are ready to cook, make 5–6 large patties out of the minced meat mixture, remembering to make a little flat indent in the centre lightly with your fingers; this will help them cook evenly and not puff up in the middle. Preheat a barbecue or griddle pan. Place on the hottest part of the grill and cook for 4–5 minutes, or until a little crisp and charred, then turn over and cook for another 1–2 minutes. Place 1 slice of the tomato on the base of each bun, top with a burger, a generous dollop of the yogurt, which should start oozing with the heat, and some onions. Place on the lid and eat immediately.