In a small bowl, combine Soy Sauce, Sake, sugar and Mirin. Set aside.

Heat the oil in a frying pan over medium-high heat. Add the chicken and cook for 4-5 minutes. Flip the chicken, reduce the heat to medium, and continue cooking on the other side. Add the peppers to the same pan and cook and stir for 3-4 minutes.

When the chicken is cooked through, add the sauce to the pan and cook until the sauce is reduced and the chicken is coated in the sauce.

Cut the cooked chicken into 1/2" thick (13 mm) slices.

Put Steamed Rice on one side of a Bento box, and sprinkle with Furikake. Place a lettuce leave as a divider between rice and chicken. Put sliced chicken and Shishito peppers over the lettuce. Then add tomatoes on the side.