Follow these steps carefully to construct the perfect Spicy Tuna Rice Bowl. The process is broken down into preparing each component before the final, satisfying assembly.

Step 1: Prepare the Sushi Rice

1. Rinse the Rice: Place the short-grain rice in a fine-mesh sieve. Rinse it under cold running water, gently swishing the grains with your fingers. Continue until the water runs clear. This crucial step removes excess surface starch, preventing the cooked rice from becoming gummy.
2. Cook the Rice: Transfer the rinsed rice to a medium saucepan and add the 2 ½ cups of cold water. Bring to a boil over high heat. Once boiling, immediately reduce the heat to the lowest possible setting, cover with a tight-fitting lid, and let it simmer for 15 minutes. Do not lift the lid during this time.
3. Rest the Rice: After 15 minutes, remove the pot from the heat but keep the lid on. Let the rice steam for another 10 minutes. This allows the moisture to distribute evenly, resulting in perfectly fluffy grains.
4. Prepare the Seasoning: While the rice is resting, combine the rice vinegar, sugar, and salt in a small bowl. Microwave for 30 seconds or warm gently in a small saucepan, stirring until the sugar and salt are completely dissolved.
5. Season the Rice: Transfer the hot, cooked rice to a large, non-metallic bowl (wood or glass is best, as metal can react with the vinegar). Drizzle the vinegar mixture evenly over the rice. Using a rice paddle or spatula, gently “cut” and fold the seasoning into the rice with a slicing motion. Avoid stirring, as this can mash the grains. Continue until the rice is glossy and has cooled to just above room temperature.

Step 2: Prepare the Spicy Tuna

1. Cube the Tuna: Pat the sushi-grade tuna loin dry with a paper towel. Using a very sharp knife, slice the tuna into small, uniform ½-inch cubes. Place the cubed tuna in a medium bowl and chill in the refrigerator while you prepare the sauce.
2. Make the Spicy Sauce: In a separate small bowl, whisk together the Kewpie mayonnaise, Sriracha, soy sauce (or tamari), and toasted sesame oil. Continue whisking until the sauce is smooth, creamy, and a consistent pale orange color. Taste and adjust the Sriracha for your desired spice level.
3. Combine: Gently pour the spicy sauce over the chilled tuna cubes. Use a rubber spatula to carefully fold the tuna into the sauce until every piece is evenly coated. Be gentle to avoid breaking up the delicate tuna pieces. If using, fold in the masago or tobiko now.

Step 3: Assemble the Bowl

1. Create the Base: Divide the seasoned sushi rice evenly among four serving bowls, creating a generous bed for the toppings.
2. Add the Tuna: Spoon a hearty portion of the spicy tuna mixture into the center of each rice bowl.
3. Arrange the Toppings: Artfully arrange the sliced avocado, cucumber, and cooked edamame around the spicy tuna. There’s no right or wrong way, but placing each topping in its own section creates a visually appealing presentation.
4. Garnish and Serve: Sprinkle the bowls generously with sliced scallions and toasted sesame seeds. Finish with a scattering of shredded nori or a dash of furikake. Serve immediately, with a side of pickled ginger and extra soy sauce if desired.