Combine rib eye with cornstarch and mix until beef is well coated. Let sit at room temperature for 30 minutes.
Heat 1 tablespoon oil in a medium saucepan over medium heat. Add sliced onion and saute until the onions are softened, about 5-6 minutes.
Add marinated beef and cook until browned. Add dashi, sake, mirin, soy sauce, sugar, ginger, and garlic. Bring to a simmer, reduce the heat to low, and continue to cook for 10-15 minutes. The liquid should barely be bubbling. Season with salt and white pepper.
Portion rice into 4 bowls. Top with beef mixture, a fried egg, green onions, pickled ginger, and togarashi. Serve immediately.
Notes
Dashi is a type of Japanese stock made from kelp (seaweed) and bonito flakes. You can buy instant dashi powder from Asian markets or make it at home. Alternatively, substitute with chicken broth instead.You can cook the beef the day before and let it sit in the cooking juices overnight. Reheat gently before serving.