Cut the beef into 1 cm (½ inch) wide strips, discarding any areas of excess fat. Place in a non-metallic bowl.
Crush the oregano and cumin seeds using a pestle and mortar and mix with the sugar, garlic and lime rind and juice. Add to the beef and mix until well combined. Cover loosely and leave to marinate in the refrigerator for 1 hour.
Heat the oil in a frying pan or wok and fry the onions and peppers, stirring frequently, for 20 minutes until very soft and beginning to colour. Drain the vegetables to a plate and wipe out the pan. Fry the beef, in batches, for 5–8 minutes per batch, turning, in the remaining oil until golden on all sides, adding the cooked meat to the plate. Return the peppers, onions and meat to the pan and heat through briefly. Add any marinade left in the bowl and cook, stirring, for about 5 minutes until browned.
Serve on the warmed tortillas with the lettuce, along with soured cream and sweet chilli sauce.