Slice 1/2 a sweet onion thin, just over half or a quarter onion. Then cut crosswise to make a fine dice.

Place in a fine sieve and rinse well under cool water. Shake out and squeeze out excess water with a paper towel. Place in a mixing bowl or container.

Add drained tuna and break apart tuna well with a fork. Add enough Mayonnaise to moisten well bit mixture is still firm.

Cut crusts off slices of white toast. Butter both slices well.

Cover with other slice of bread and wrap in parchment paper. Press under a heavy wooden cutting board for 5-10 mins. Cut in half.

Make bento side dish like Asian cucumbers and arrange in bento box.