Thinly slice the ginger. Trim the radish sprouts and cut them in half. Remove the stems from the cherry tomatoes.

Make shallow cross cuts on the skin side of the flounder fillets.

In a frying pan, combine sake, mirin, water, and ginger. Bring to a boil over medium heat. Place the flounder skin-side up and spoon the simmering liquid over the fish until the surface changes color. Cover and simmer on low heat for 5 minutes.

Uncover the pan and dissolve the miso into the sauce. Heat over medium until boiling, then cover and simmer on low for another 5 minutes. Spoon the sauce over the fish and simmer for an additional minute to thicken.