Make the rice: Make the rice on the stovetop or in an Instant Pot. When it’s done, give it a few shakes of rice vinegar and stir in salt to taste. (Or, make the rice in advance and reheat it on the stovetop with a splash of water.)
For the poke: Slice the tuna into 1-inch cubes. Mince the onion. Thinly slice the green onions. Mix them in a bowl with the soy sauce, sesame oil, garlic, ginger, kosher salt and Sriracha.
For most authentic flavor marinate in refrigerator for 15 minutes to 1 hour. Taste and add a sprinkle of salt before serving. (Get this in the fridge while the rice boils and you prep the veggies, and you can eat when it’s all done.)
Prepare the vegetables: Thinly slice the red cabbage. Place it in a small bowl and add a few shakes of rice vinegar and a few pinches salt. Thinly slice the cucumber and radishes, and salt them with a few pinches of salt.
Assemble the poke bowls: Place the rice in the bowl and top with the poke and vegetables. Garnish with sesame seeds and greens.