Whisk together lime juice, lime zest, brown sugar, vegetable oil, fish sauce, sriracha, red pepper flakes, ginger, and garlic to make marinade.
Marinate steak with half the marinade for 20 minutes, then sear in butter for 4 minutes per side. Rest and slice thinly.
Combine lettuce, cucumber, tomatoes, herbs, and red onion in a large bowl. Toss with remaining dressing, add sliced steak, and top with peanuts if desired.
Notes
For extra flavor, let the steak marinate longer and add more herbs for freshness.