Prepare rice for the sumeshi. Place rice in a large bowl. Pour water to cover the rice. Wash it with your hand moving in circular motion then quickly discard the water. Repeat the process 5 to 6 times until water becomes clear. Drain well. Alternative way: Place rice in a fine mesh colander. Rinse under running water. Use your hand, moving in a circular motion to rinse thoroughly until water becomes clear. Drain well.
Transfer the rice to preferably a cast iron pot with a heavy lid or something similar. Pour water and make the rice leveled. Place kombu sheet on top of the rice (submerge in the water) and cover with the lid.
Cook over slightly higher than medium heat. In about 10-15 minutes, it starts almost boiling (you will see the bubbles.) At this point, reduce the heat to low and continue to simmer for 10 minutes. After the 10 minutes, turn off the heat. Keep the lid on and let it stand for 10 minutes.
While rice is still cooking, start preparing grapefruit. Slice off the top and bottom end of grapefruit. Then place it cut side down and carefully slice off the entire peel & membrane. Once the entire flesh is exposed, cut out the segments one by one. Do this over a bowl to catch the segments as well as the juice. After all the segments are cut out, squeeze the remaining to get the juice out in the bowl. Grab the segments from the bowl and cut them into bite size pieces then transfer to another bowl. Set it aside for later.
Transfer 2 tbsp of freshly squeezed grapefruit juice into another bowl (save the remainder of grapefruit juice for later.) Add rice vinegar, sugar, and salt. Whisk together until well blended. Set aside. You may see the sugar and salt are not quite dissolved which should be fine once it is mixed with warm rice.
When the rice is done, remove the kombu sheet and transfer the cooked rice to a large shallow bowl (preferably wooden sushi bucket for soaking up excess moisture.) Add the grapefruit sushizu and mix with a rice paddle. To do so, move in a gentle cutting motion. As the rice is getting mixed, use a fan (or whatever that comes in handy) to let the rice cool down until ever so slightly warm like your body temperature. Sushi rice tastes better when it is not completely cold.
At this point, start dicing avocado roughly the same size as the grapefruit segments. Place the diced avocado in a bowl and pour the remaining grapefruit juice. Toss lightly.
To serve, place the sushi rice (sumeshi) in a large bowl or serving bowl. Top with grapefruit, avocado, radish microgreens or sprouts and shredded nori (optional.) Pour soy sauce along with wasabi over and enjoy!
Notes
Kombu sheet adds umami and depth to the overall taste of rice.
Mix toasted sesame seeds into sumeshi for accented taste.
Make rice using your rice cooker. If you use Japanese rice cooker (such as Zojirushi brand), you may have a measuring cup that came with the rice cooker. 1 ½ cups of uncooked rice = 2 cups of the rice cooker measuring cup. Use the sushi rice setting and prepare it accordingly.