Sprinkle salt all over the steaks and let them sit at room temperature for 30 minutes. Mix the soy sauce, sake, and mirin in a bowl and set aside.
Heat a frying pan over medium-high heat with oil. Cook the steaks for 2–3 minutes until nicely browned, then turn them over and cook for another 2 minutes, or until they reach your desired doneness.
Pour the soy sauce mixture over the steaks and cook for about a minute, coating them well. Remove the meat from the pan and let it rest. Meanwhile, reduce the sauce slightly in the pan. Add the butter, stir to combine, and remove from the heat.
Cut the steak into bite-sized pieces. Put steamed rice in a bowl, layer lettuce over the rice, and place the meat on top. Spoon the sauce over the meat and garnish with thinly sliced green onion.