Preheat the grill to high. Mix together the lamb, white onion, herbs and spices and season with salt and pepper. Divide the mixture into 4 and, using your fingers, squeeze it around skewers to form long sausage shapes.
Grill the kofta kebabs for 10-12 minutes, turning occasionsally, until well browned but still a little pink in the centre.
Make the chilli yogurt: combine the yogurt, chillies, coriander, garlic, lime juice and sugar, and season to taste.
briefly warm the tortillas for a few seconds on each side in a dry, non stick frying pan, to make them soft and pliable. Place a kebab in the centre of each tortilla and squeeze around the kebab, Pulling it off the skewer. Scatter over a few onion rinds, and drizzle over the yogurt. Serve with tomato wedges, crisp lettuce leaves and a wedge of lime to sqeeze over.