Season the flank steak with soy sauce, fish sauce, brown sugar, lime juice, garlic powder, and black pepper. Let it marinate for at least 30 minutes at room temperature or up to 2 hours in the refrigerator.
Heat the vegetable oil in a grill pan or skillet over medium-high heat.
Remove the steak from the marinade and cook for about 4-5 minutes on each side for medium-rare, or until desired doneness.
Let the steak rest for 5 minutes before slicing it thinly against the grain.
While the steak is resting, prepare the Nam Jim Jaew sauce by combining fish sauce, lime juice, rice powder, brown sugar, chili flakes, garlic, and cilantro in a bowl. Mix well until the sugar dissolves.
Serve the sliced beef with the Nam Jim Jaew sauce drizzled on top or on the side for dipping.