Put rice in a bowl, fill it up with cold water, and rub in a circular motion. The water will become opaque which means the rice still needs to be cleaned. Strain the water out and repeat until the water is clear.
Place the cleaned rice, kombu, and the amount of water required into a rice cooker and allow the rice to cook.
Heat a pan, with ½ tablespoon of neutral oil, over medium-high heat. Add the spinach and cook until wilted, about 2 minutes. Transfer to a medium bowl.
Season the tuna with salt and pepper on all sides. Spray the pan used for the spinach with non-stick spray and heat on medium-high heat. Saute the tuna until each side turns golden and the fish is cooked through, about 6 to 8 minutes.
Shred the tuna and allow to cook until cooked through, about 3 minutes. Remove from the heat. Transfer to the bowl with the spinach. Add the scallion and toss until combined.
When the rice is done cooking, remove from the rice cooker and place inside a non-metallic bowl. Discard the kombu.
In a small bowl, combine rice vinegar, sugar, and salt. Add the vinegar mixture while the rice is still hot. Take a rice paddle and fold in the rice vinegar. Continue to fold and slice the rice until it has cooled down. Cover with a wet towel to keep the rice nice and moist.
To assemble the omusubi, split the rice into 6 portions. Take half of one of the portions and place on a large sheet of plastic wrap. Shape into a disc with a deep divot in the center. Place hearty serving of the filling in that divot, but don’t overfill this because it will make it more difficult to wrap.
Note: When working with the sushi rice, it is extremely important that you keep your hands moist. Keep a bowl of water nearby.
Top with the other half of the rice and seal in the filling, shaping into a rough ball. Wrap with the plastic wrap and place a moderate amount of pressure to shape into a triangle. If you are not eating this immediately, keep it wrapped in the plastic wrap and place in the refrigerator. These can be stored in the refrigerator for up to 1 week.
Repeat steps 9 and 10 with the remaining portions. Serve with a piece of nori.