Over medium-high heat, boil miso and salt in 2 cups water until miso is diluted in the water, about 5 minutes. Add brown sugar and let reduce to about 1/4 cup syrup. Remove from heat and let cool.
Lay cod fillets in a nonreactive shallow container such as a glass baking dish. Spoon about 1 tablespoon of the reduction over each fillet, cover and let sit in the refrigerator for about 2 hours.
Preheat oven to 375 degrees F. Bake the cod until it flakes, about 5 to 7 minutes. Remove from oven and let rest.
Garnish with chives.
Note: In the episode this dish was used as part of the bento box of 5 cooked tasting size entrees in conjunction with Chicken Yakitori with Scallions; Shrimp Tempura; Gyoza: and Korokee.