Slice the flank steak against the grain into thin strips.
In a bowl, combine soy sauce, fish sauce, lime juice, brown sugar, garlic powder, and black pepper.
Add the sliced beef to the marinade, ensuring all pieces are well coated. Let it marinate for at least 30 minutes, or up to 2 hours in the refrigerator for more flavor.
Heat the vegetable oil in a large skillet or grill pan over medium-high heat.
Once hot, add the marinated beef strips in a single layer, cooking for about 3-4 minutes per side until browned and cooked to your desired doneness.
Remove from heat and let rest for a few minutes.
While the beef is resting, prepare the Nam Jim Jaew sauce by mixing fish sauce, lime juice, rice powder, chili flakes, sugar, and chopped cilantro in a small bowl. Stir until well combined.
Slice the beef into bite-sized pieces and serve with the Nam Jim Jaew sauce on the side for dipping.