Ingredients:

• 3 cups of  hot/warm cooked short-grained rice 
• 1 can tuna drained 
• 2 tablespoon sriracha or hot sauce 
• 1 tablespoon mayonnaise salt and pepper to taste 
• 3 tablespoon green onion finely chopped 1 teaspoon lemon juice 
• 2 tablespoon chili pepper flakes optional 
• 1 avocado sliced

Equipment:

• Onigiri mold (optional)

Preparation:

1. Combine tuna, hot sauce, mayonnaise, salt and pepper, green onion, lemon juice, and pepper flakes into a bowl and mix thoroughly. Adjust seasonings to taste.
2. Place into the refrigerator until ready to use, at least 1 hour. (The longer it sits, the more flavorful the tuna will be).
3. When ready to assemble, using an onigiri mold, add about 1 heaping tablespoon of warm rice to the mold, press down lightly with a spoon.
4. Add 1 tablespoon of spicy tuna filling over top of the rice.
5. Finish by adding another 1 tablespoon of rice on top of the filling to encase it. Press the mold firmly to retain the shape of the rice ball. Onigiri is just a rice ball. There’s no “right or wrong” way to shape the onigiri. Common shapes are triangles or round balls. So have fun and play around with the shapes!
6. Open the mold and remove the onigiri. 
7. Dip the exterior of the onigiri into some sesame seeds and furikake if you wish, and wrap it with a piece of nori.

Tips:

• Depending on the size of your mold, you may need to adjust how much filling and rice you need. The bigger, add a bit more. The smaller, add a little less.
• Be sure to use hot/warm rice as cooled rice will not stick together well!

Source: https://www.siftandsimmer.com/spicy-tuna-onigiri/