Season the beef cubes with salt and pepper and place them in a bowl or resealable plastic bag. Whisk together the olive oil, lime juice, chilli powder, cumin and garlic. Pour over the beef cubes and turn to coat them well. Let stand for 30 minutes at room temperature.
Preheat the grill or barbecue. Thread the beef onto 4 skewers.
Cook the beef under the grill or on the barbecue until tender, 7 to 8 minutes, turning frequently. Grill the onion slices alongside until soft and lightly charred. Towards the end of cooking, wrap the tortillas in aluminium foil and warm gently under the grill or on the barbecue, away from the direct heat.
Chop the barbecued onion coarsely. Serve the kebabs with the warmed tortillas, onion, tomato, lettuce and soured cream.