Stir the lime juice, fish sauce, sugar, sliced chilies, and garlic together in a small bowl until the sugar dissolves. Taste the dressing and adjust the lime, fish sauce, sugar, or chilies so it tastes sour, salty, sweet, and spicy; add the water only if it tastes too strong.

Slice the red onion, cucumber, and tomatoes, then pick the cilantro, mint, and Thai basil leaves. Keep the prepared vegetables and herbs chilled while the beef cooks.

Pat the steak dry and season both sides with kosher salt and black pepper. Heat a grill, grill pan, or heavy skillet over high heat, adding the neutral oil if using a skillet.

Cook the steak for approximately 3 to 5 minutes per side, depending on its thickness, until medium-rare. The center should reach about 130 to 135°F (54 to 57°C) after resting.

Transfer the steak to a plate and let it rest for 5 to 10 minutes. Slice it thinly across the grain so the muscle fibers are shortened and every piece stays tender.

Toss the sliced beef with the red onion, cucumber, tomatoes, cilantro, mint, and Thai basil. Add the dressing gradually and toss gently, then serve immediately over salad greens or on its own.

If using toasted rice powder, stir it in at the end along with scallions or extra sliced chilies. The rice powder adds a lightly nutty toasted flavor associated with Thai nam tok-style beef salad.