01 - Rinse sushi rice under cold water until water runs clear. Combine rice and water in a saucepan; bring to a boil, then cover and simmer over low heat for 15 minutes. Remove from heat, keep covered, and let stand for 10 minutes. Whisk rice vinegar, sugar, and salt until dissolved, then gently fold into cooked rice. Allow rice to cool to room temperature before assembling bowls.
02 - In a medium bowl, whisk together soy sauce, sesame oil, sriracha, mayonnaise, rice vinegar, honey, sesame seeds, and spring onion until well combined. Add diced tuna and toss gently to coat evenly. Cover and refrigerate for at least 10 minutes to allow flavors to meld.
03 - Divide seasoned rice among four serving bowls. Arrange marinated tuna, sliced avocado, cucumber, carrot, edamame, and nori strips on top of rice in sections. Garnish with pickled ginger, sesame seeds, chili slices, and microgreens or radish. Serve immediately, mixing ingredients just before eating.