Rinse the sushi rice under cold water in a fine-mesh sieve until the water runs mostly clear. Add the rice and water to a rice cooker or saucepan and cook according to the package directions.

Let the cooked rice rest, covered, for 10 minutes. Fluff gently with a rice paddle or fork. While warm, mix in the salt, rice vinegar, and sugar if using. Let cool slightly until comfortable enough to handle.

In a small bowl, combine the drained tuna, mayonnaise, sriracha, soy sauce, sesame oil, rice vinegar, and green onions. Stir with a fork until creamy. Mix in furikake and sesame seeds if using.

Prepare a small bowl of water with a pinch of salt. Wet your hands lightly before shaping each rice ball.

Scoop about ⅓ cup of warm rice into one hand and flatten it gently. Add about 1 tablespoon of spicy tuna filling in the center.

Cover the filling with a little more rice. Press and shape gently into a round, oval, or triangle, ensuring the filling is sealed inside. Repeat with the remaining rice and filling.

Sprinkle with furikake or sesame seeds if desired. Wrap one nori strip around each rice ball.

Serve immediately with soy sauce, extra sriracha, spicy mayonnaise, pickled ginger, or a side of cucumber salad.