Chill the beef briefly. Place the steak in the freezer for 15 minutes. This helps it sear better and makes slicing easier later.
Mix the dressing. In a bowl, whisk lime juice, fish sauce, soy sauce, brown sugar, sesame oil, garlic, ginger, and chili.
Adjust sweet and heat levels. The taste should be bright, spicy, and slightly sweet.
Prep the salad base. Add cucumber, tomatoes, red onion, lettuce, mint, cilantro, and sliced chili to a large bowl. Keep the herbs whole or roughly torn for best texture.
Season the steak. Pat the steak dry.
Rub with oil, then season with salt and pepper on both sides.
Sear the steak hot and fast. Heat a skillet or grill pan over high until smoking. Sear steak 3–4 minutes per side for medium-rare, depending on thickness. Avoid moving it around too much.
Rest the meat. Transfer steak to a plate and rest 10 minutes.
This keeps it juicy and tender.
Slice thinly against the grain. Cut the steak into thin slices across the grain to keep it tender. If the slices seem wide, cut them in half lengthwise.
Toss with dressing. Drizzle half the dressing over the salad bowl and toss gently. Add the beef and more dressing to taste.
You want everything lightly coated, not drenched.
Finish and serve. Sprinkle with peanuts if using. Taste and adjust lime, salt, or heat. Serve right away with lime wedges.