There are many different fillings for a musubi (a rice sandwich) in addition to the popular Spam Musubi.  Today for lunch, we had a tuna musubi. 

An important piece of equipment for making musubi is a rice press, which can be bought on Amazon for less than $10.

Here is how we made 3 servings:

1 can tuna in oil

Mayonnaise (preferably Kewpie, a Japanese brand0

Furikake (Japanese sesame seasoning)

2 cups uncooked Japanese medium grain rice (like Cal-Rose or Nishiki)

Cook the rice in a rice cooker.

Mix together in a small bowl the tuna and mayonnaise into a smooth mixture.

Wet down a wooden board.  Fill the rice press about 2/3 full with the cooked rice and press down hard  to form a compact half-inch thick layer.  Sprinkle the rice layer generously with Furikake.  Top with the tuna and mayonnaise.  Sprinkle the tuna layer generously with Furikake.  Fill the rest of the rice press with cooked rice and press down hard to form another compact half-inch thick layer.  Remove the musubi from the rice press, wrap with half a sheet of nori, and eat.

The musubi can be made in advance just like a sandwich.  In that case, do not wrap with the nori until you are ready to eat, so that the nori does not get damp and soft.