Soak the clams (still in their shells) in lightly salted water for about an hour to remove any sand and impurities.
Place the water and kombu in a pan, add a third of the clams from (1) and heat. Once the water has boiled, remove the scum, place the clams in a colander and keep the liquid to one side. Dry the clams and cool in ice water, then remove from their shells.
Place half of the remaining clams into the liquid from (2), and repeat step (2), then repeat once more for the final third. This gives a clam stock with three times the flavour.
Dissolve the miso into the stock from (3). Add in all of the clams and heat until right before boiling. Then pour over bowls of rice, garnish with asatsuki (chopped 1cm long), and sprinkle with shichimi to taste.