Mix together the chilli oil, chilli powder, paprika, sugar and the lime rind and juice in a shallow non-metallic dish. Add the steak to the chilli mixture and stir until well coated, then cover with clingfilm and leave to marinate in the fridge for 1-2 hours.
To make the chilli sauce, heat the oil in a large pan and fry the onion for 6-8 minutes until golden. Stir in the garlic, chilli, paprika, cumin and coriander and cook for 1 minute, stirring. Add the tomatoes and lemon juice and season to taste. Simmer gently for 10-15 minutes until slightly reduced and thickened.
To prepare the steak filling, heat the sunflower oil in a wok or large frying pan. Add the sunflower oil and sauté the onion and peppers for about 5 minutes until softened and just beginning to catch around the edges. Tip in the marinated steak and continue to sauté for 3-4 minutes until the steak is just tender and cooked through.
Use the steak and pepper mixture to fill the tortillas, then roll up and lay side by side in an ovenproof dish. Spoon over the tomato sauce and scatter the cheese on top. Bake for 15-20 minutes until bubbling and golden brown. Serve straight to the table with a separate bowl of salad and allow guests to help themselves.