Whisk together lime juice, garlic, sugar, fish sauce and sesame oil in a jug. Place steak in a large snap-lock bag. Drizzle with half the dressing. Place in fridge for 15 minutes to develop the flavours.
Preheat a pan or barbecue to high heat. Cook steak for 2-3 minutes on each side for medium-rare or until cooked to your liking. Transfer to a plate covered loosely with foil for 5 minutes to rest.
Place lettuce, tomato, cucumber, onion and your choice of herbs in a large bowl. Thinly slice steak across the grain and add to salad. Drizzle with remaining dressing and gently toss to combine.