In a small bowl, combine 1 tablespoon of ponzu, 1/4 teaspoon sugar, and a pinch of kosher salt with the daikon, and stir to combine. In another bowl, combine another tablespoon of ponzu, sugar, and a pinch of salt with the cucumber, and stir. Set the daikon and cucumber aside.

Give the tuna a quick rinse and pat dry. On a clean, dry cutting board and with a sharp knife, dice the tuna into 1/4-inch cubes.

In a medium-sized bowl, combine the tuna, sesame oil, chives, salt, sesame seeds, and soy sauce. Give a quick stir to thoroughly coat the tuna.

In separate strainers, drain the daikon amd cucumber. To serve, pack a ring mold with a quarter of the tuna mixture. Gently remove the mold. Carefully spoon about a tablespoon of the cucumbers on one half of the tuna, and another tablespoon of the daikon on the other half. Sprinkle some toasted sesame seeds to garnish. If you have black sesame seeds, add a small pinch of that also for contrast.