Heat the grill to high and scatter the beef, onions and pepper pieces on a large baking tray. Mix the oil and seasoning in a small bowl, then rub over the meat and vegetables with your hands. Grill for 10 mins, turning halfway, until everything is cooked through and the peppers and onions are starting to blacken at the edges. Heat the tortillas in the microwave or under the grill according to pack instructions.
Meanwhile, tip the tomatoes into a sieve and leave to drain from their juice for 5 mins. Discard the juice, mix the flesh with most of the chopped coriander and season to taste. Wrap the spicy beef and vegetables, salsa and a little of the remaining coriander in the tortillas, topping with a spoonful of soured cream or yogurt, if you like.