Combine all of the sauce ingredients in a small bowl.
Cut the thawed tuna into bite-sized chunks, which will be easier to do if it is still very slightly frozen.
Peel the cucumber and remove its seeds with a spoon, then chop it into bite-sized pieces.
Slice up the avocado, onion, radishes, and green onions, then cut up the seaweed with kitchen shears.
Assemble your Poke bowls: Scoop cooked rice into each bowl. Top bowls with chopped tuna, seaweed, and veggies. Serve with sauce to taste.
Notes
Spice these homemade Poke bowls up with different sauces, if desired. Try teriyaki, for instance, or for a sweet & spicy take substitute a little honey & sriracha for the lime juice & crushed red pepper in the sauce recipe here. You can also top with sesame seeds for additional flavor & crunch, if you have any on hand.