Slice the flank steak against the grain into thin strips.
In a bowl, mix soy sauce, fish sauce, lime juice, brown sugar, garlic powder, black pepper, and red pepper flakes.
Marinate the beef in this mixture for at least 30 minutes, or up to 2 hours in the refrigerator.
Heat the vegetable oil in a skillet over medium-high heat.
Add the marinated beef strips in a single layer, cooking for about 3-4 minutes on each side until browned.
Remove from heat and let rest for a few minutes.
Prepare the Nam Jim Jaew sauce by combining fish sauce, lime juice, rice vinegar, brown sugar, chili flakes, green onions, and toasted rice powder in a small bowl.
Mix well until the sugar dissolves.
Slice the beef into bite-sized pieces and serve it on a platter.
Drizzle with the Nam Jim Jaew sauce, garnish with fresh cilantro, and serve with lime wedges on the side.