In a large bowl, whisk together the water, soy sauce, sugar, sesame oil, garlic and onion powders, and Korean red pepper flakes until the sugar has dissolved. Add the sliced rib eye and let marinate for 1 hour.
In a large cast-iron pan, heat the vegetable oil over high heat. Drain the meat, discarding the marinade. Working in batches, cook the meat, flipping once, until caramelized and golden, 1 minute per side. Transfer to a platter.
Garnish the bulgogi with the scallions, sesame seeds and more pepper flakes, then serve with steamed rice and kimchi.