Take the steak out of the fridge about 20 minutes before cooking to bring it to room temperature. Pat it dry and season lightly with salt and pepper. Heat oil in a skillet over high heat until hot.
Sear the steak for about 2–4 minutes per side, depending on thickness and preferred doneness. Medium-rare works best for tenderness and juiciness. Remove from heat and let it rest for at least 10 minutes.
In a small bowl, mix fish sauce, lime juice, sugar, minced garlic, and sliced chili. Stir until the sugar dissolves completely. Taste and adjust sweetness or acidity if needed.
After resting, slice the steak thinly against the grain. This step ensures the meat stays tender and easy to chew. Keep any juices released and add them to the salad for extra flavor.
In a large bowl, combine tomatoes, cucumber, red onion, cilantro, and mint. Add the sliced beef on top along with any resting juices. Pour the dressing over everything.
Gently toss to coat all ingredients evenly with the dressing. Serve immediately while the beef is still slightly warm for the best flavor contrast.