Put the lamb, chopped coriander and half the onion into a food processor bowl. Tip the seeds into a pestle and mortar and grind roughly. Add all but 1 teaspoon to the mince mixture, season then blitz together. Alternatively, finely chop half the onion and coriander then mix into the mince with the spices.
Shape the lamb mixture into 4 thick burgers, arrange on a baking sheet, cover and chill until required. Wrap the remaining sliced onions and chill until required.
Crush the garlic and mint together in a pestle and mortar, add the remaining crushed seeds, the yogurt and some pepper. Spoon into a small dish, cover and chill until required.
When ready to serve, grill or barbecue the burgers for about 10 minutes, turning once or twice until browned and cooked right through. Slit and toast the cut sides of the rolls then add the lettuce, burgers, spoonfuls of the yogurt mix and the remaining onions. Serve with chunky chips, if liked.