Cut mizuna into bite-sized pieces. Slice shiitake mushrooms thinly, separating stems and caps. Slice green onion diagonally and mince ginger.

Rub clams together in a bowl of water to clean their shells, then drain.

Heat sesame oil in a pot over medium heat. Add green onion and ginger, sauté until fragrant. Add clams and shiitake mushrooms, stir briefly, then add water. Simmer until clam shells open, skimming off any foam.

Add seasoning ingredients (sake, salt, pepper, miso, gochujang) and mizuna. Stir lightly and simmer on low heat for 2 minutes.