Gather the ingredients.

Pour rice into a heavy pot and add water; swirl to allow rice to settle. Bring to a simmer over medium-high heat; do not stir. Reduce heat to low, cover, and continue to simmer for 15 minutes.

While rice is cooking, place tuna into a large mixing bowl and break up with your hands or a fork. Toss in red bell pepper, jalapeno, 1/4 cup green onions, rice vinegar, lemon juice, soy sauce, Sriracha, and sesame oil. Mix with a fork until thoroughly combined. Turn off heat and let rice sit, covered, for 10 minutes.

Fluff rice with a fork to separate the grains and break up any large clumps; transfer into the mixing bowl. Mix thoroughly with a spoon until all the ingredients are evenly incorporated. Taste and adjust seasoning if needed. Serve warm, at room temperature, or cold like a rice salad, topped with red pepper flakes and 1 teaspoon green onion.