Cut mizuna into 3cm pieces and chop shibazuke pickles.

In a pot, combine canned clams and seasoning ingredients. Heat on medium until boiling, then simmer on low until the liquid is almost gone.

Place a sheet of nori on plastic wrap with a corner facing you. Add 1/4 of the rice in a square shape, followed by half the clam mixture, mizuna, and shibazuke. Top with another 1/4 of the rice. Fold the nori corners inward and wrap tightly with plastic wrap.