1. Remove the flesh from the avocado. Peel and roughly dice the cucumber. Blitz the avocado, cucumber, sour cream, spring onions, dill and lime juice into a smooth purée in a blender. Mix in the chilli sauce and salt to taste. Pour the sauce into a bowl, and leave covered in the fridge until ready to serve. Allow to return to room temperature before serving.
2. Lay out metal, wood or bamboo skewers. Wood and bamboo skewers must be soaked in water for at least 30 minutes.
3. Prepare the barbecue for medium direct heat 180°-230°C.
4. Mix the seasoning ingredients in a bowl. Add the steak cubes, and mix them in until evenly covered by the spices. Thread the meat onto the skewers, alternating with the tomatoes. Thinly coat the meat and tomatoes with oil, covering evenly.
5. Brush the cooking grate clean. Grill the kebabs over a medium direct heat with the lid closed until the desired level of cooking has been reached, approx. 8 minutes for pink/red (medium-rare). Turn the kebabs occasionally. Remove from the barbecue.
6. Serve the kebabs hot with the sauce.