Sesame coated fish cakes are one of my favourites, and a good way to use up leftover mashed potatoes from dinner the night before. The recipe is below, you can kind of adapt it to how much time you have got when you are making your bento, and how adventurous you want (or don't want) to be. They feature in this bento on a bed of baby lettuce leaves, with fresh tomato halves and some chopped mango.
Sesame coated fish cakes:
~1 1/2 cups leftover mashed potato (you can vary the amount if you have a bit more or less is ok)
1 can of tuna, drained
salt and pepper to taste
and any (or all) of the following:
finely chopped onion
finely chopped red pepper
small amount of grated cheese
Mix everything together, shape into flattened balls and roll in sesame seeds. Cook on both sides in a little olive oil. If you want you can place them on a paper towel before putting into your bento (I like to do this to remove any excess oil). These fish cakes are not only tasty, they look good as well!