Prepare rice using your preferred method (stovetop, rice cooker, or Instant Pot) and briefly allow to cool.
While the rice is cooking, combine mayonnaise, rice vinegar, and sesame oil in a small bowl. Add furikake to taste, if desired. Stir until fish has flaked and ingredients are completely combined. Store in the refrigerator until ready to use.
While rice is still warm (but not hot), dampen one clean hand and cover it with salt.
Place roughly 1/4 cup of rice in your hand.
Place approximately 2 teaspoons of tuna mayo filling at the center of the rice.
Wrap rice around the filling to form a ball.
Use other hand (or a mold) to shape the onigiri into your preferred shape.
If not eating them immediately, wrap onigiri in plastic (or in a damp dishcloth) and place in the refrigerator. Do not eat if stored for longer than 48 hours.
Immediately prior to serving, wrap shaped onigiri with nori and garnish with furikake, if desired.