Tear the green leaf lettuce into bite-sized pieces.

Place the frozen yaki onigiri on a microwave-safe plate, cover loosely with plastic wrap, and heat according to package instructions. Slice each onigiri in half horizontally.

Heat half of the salad oil (1 tsp) in a frying pan over medium heat. Add the sliced yaki onigiri and cook until both sides are golden brown. Remove from the pan.

In the same pan, heat the remaining salad oil (1 tsp). Add the sliced beef and stir-fry over medium heat until the beef changes color. Add yakiniku sauce and cook until glossy.