(If using steak) Make marinade:
1. Peel and chop lemongrass.
2. Place all ingredients in a blender and puree until smooth.
3. Pour marinade over the hanger steaks and allow to sit in refrigerator 2 to 8 hours.
Trim the tough ends of the watercress then wash, and dry in a salad spinner or with a paper towel
Combine mint, cilantro, radishes, red onion, and lemon zest in a medium bowl. Cover and refrigerate until ready to serve.
Make dressing: In another bowl, whisk together the sesame oil, lime juice, fish sauce, soy sauce, honey, salt and pepper. Set aside.
Preheat grill or grill pan. Remove the hanger steak from the marinade and blot to remove any excess. Grill the steak about 4-5 minutes per side for medium rare.
Remove steak and let it rest about 5 minutes.
Assemble salad: Toss the green sin the dressing (might not need all of it) and place it on a platter.
Slice steak thinly against the grain and arrange it over the greens.
Sprinkle steak with chopped peanuts (optional) and serve!