In a medium to large bowl, combine the fish sauce, garlic, minced cilantro and Thai basil stems, oil, sugar, pepper, and salt. Add the steak, and rub the marinade mixture all over the steak. Set aside to marinate overnight or, for more tender cuts, at least 20 minutes.

In a small bowl, dissolve the brown sugar or palm sugar in the hot water. Whisk in the lime juice, oil, fish sauce, chopped cilantro and Thai basil, garlic, and salt and pepper.

In a large bowl, add the mixed greens, romaine lettuce, chopped scallion, cilantro, mint, and Thai basil. Toss in the cucumbers and tomatoes right before you dress the salad.

Preheat a grill or grill pan/cast iron skillet until searing hot. Add the marinated skirt steak, and let it cook for about 1 minute. Rotate the skirt steak 45° for more grill marks, and cook for another minute.

Flip the steak and repeat these steps for the other side. Total cooking time should be about 4 minutes for medium-rare; adjust as needed for thicker cuts. Let the steak rest for 3-5 minutes.

Pour about half of the salad dressing over the salad. Toss and add more dressing to taste. Transfer to two large plates.

After the beef has rested, slice the steak against the grain into bite-size pieces and place equal portions on top of each salad. Top with chopped peanuts and Thai chili if using, and serve any remaining dressing on the side.