In a bowl, mix fish sauce, soy sauce, lime juice, sugar, and black pepper
Rub the marinade over the beef and let it rest for 20–30 minutes
Preheat a grill pan or outdoor grill over high heat
Lightly oil the grill and cook the beef for 3–5 minutes per side for medium-rare, or to desired doneness
Remove the beef from heat and let it rest for 5–10 minutes
Slice the beef thinly against the grain
In a small bowl, combine tamarind paste, fish sauce, lime juice, sugar, chili flakes, and toasted rice powder
Stir well until the sauce is balanced between spicy, sour, salty, and slightly sweet
Add cilantro and green onions to the sauce
Serve sliced beef with the dipping sauce on the side
