Season the rice. In a small bowl, mix rice vinegar, sugar, and salt until dissolved.
Gently fold into the warm cooked rice. Let it cool to just above room temperature. The rice should be slightly sticky but not wet.
Prep the tuna. Pat the tuna dry with paper towels.
Using a sharp knife, cut it into even 1/2-inch cubes. Keep it chilled until you’re ready to dress it.
Make the spicy sauce. In a medium bowl, stir together mayo, sriracha, soy sauce, sesame oil, and lime juice. Add ginger and garlic if using.
Taste and adjust the heat—add more sriracha if you like it hotter.
Toss the tuna. Add the tuna to the sauce and fold gently to coat. You want every piece glossy, not drowned. Chill for 5–10 minutes while you prep toppings.
Prep the toppings. Slice the avocado, chop cucumbers, and thinly slice scallions.
Toast sesame seeds in a dry skillet over low heat for 1–2 minutes until fragrant if you haven’t already.
Assemble the bowls. Divide the seasoned rice between bowls. Spoon the spicy tuna on top. Add avocado, cucumber, carrots or radish, and edamame as you like.
Finish with crunch and color. Sprinkle scallions, toasted sesame seeds, and nori strips or furikake.
Drizzle a little chili oil or extra sriracha if you want more kick.
Serve right away. Poke bowls are best fresh. The cooler fish against the warm rice is a great contrast, so don’t wait too long.