Place the lime leaves, shallot, garlic, herbs, vinegar and 4 tablespoons of the oil in a food processor, blend to form a thick sauce and season to taste.
Toss the onion and steaks with the remaining oil and season well. Heat a griddle pan until smoking hot, add the onion and steaks and cook for 2 minutes. Turn the steaks over and move the onions around the pan and cook for a further 2–3 minutes until cooked to your liking. Remove from the pan and keep warm.
Warm the tortillas through on the griddle pan. Cut the steaks into thick slices and divide between the tortillas with the onions. Drizzle with the yogurt and herb sauce, then sprinkle with the chilli. Roll up the tortillas neatly and serve immediately.