Cut the daikon radish into 10 cm or so, and make long, continuous slices from the perimeter (this is called katsura muki). The slices should be 1 to 1.5 mm thick.
Place the pork belly slices on top of the daikon radish slices. Place green beans or similar in the center, and roll up tightly. (If the daikon radish is too thick, it may break apart.)
Secure the roll in 2 places with toothpicks.
Put enough dashi stock to cover the daikon radish rolls in a pot with shiro-dashi, sake, mirin and dark soy sauce. Add the daikon radish rolls.
When it comes to a boil, remove the scum diligently. Turn the heat down to low, cover with a small lid or a piece of aluminum foil that sits right on top of the pan contents (a drop lid or otoshibuta), and simmer for about 15 minutes. Turn the heat off and let it cool down as-is, so that the flavors permeate the rolls.
Cut the rolls in half, take out the toothpicks and arrange on serving plates. Spoon over some of the cooking liquid to finish.