For the teriyaki chicken:
Heat oil in a skillet. Add the chicken thigh, cover, and cook until both sides are golden brown.

Wipe off excess fat, then add the combined soy sauce, honey, and sake. Simmer until the sauce thickens.

For the rice:
Line an 8 cm (about 3-inch) ramekin with plastic wrap. Press in half the rice to form a patty. Repeat with the remaining rice to make two patties.

Lightly oil a skillet and cook the rice patties on both sides until golden. In the same pan, cook the kabocha slices until tender.

To assemble the rice burger:
Spread mayonnaise on one side of a rice patty and, if desired, whole grain mustard on the other patty.

Layer lettuce, kabocha, tomato, and sliced teriyaki chicken between the rice patties to make a sandwich.