Pre-heat the grill to high; Mix together the lamb, onion, herbs and spices and season with salt and pepper; Divide the mixture into four and squeeze it around skewers to form long sausage shapes.
Grill the kofta kebabs for 10-12 minutes, turning ocasionally until well browned but still a little pink in the centre.
Make the dip by combining the yoghurt, 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, or about 10 seconds in a microwave, to make them soft; Place a kebab in the centre of each tortilla, pulling it off the skewer; Scatter over the onion rings, drizzle over the chilli-yoghurt and serve.