How to Make It
Place water, tea bags, star anise (if using), ginger, and 2 tablespoons of the soy sauce in a large pot. Cover with a tight-fitting lid; bring to a boil over high. Remove from heat; let stand 5 minutes.
Remove and discard tea bags from tea cooking liquid. Measure 1/2 cup cooking liquid (with no star anise or ginger pieces) into a small bowl; set aside. Bring remaining cooking liquid in stockpot to a very gentle simmer over low. Add fillets to pot; cover and simmer until fish just flakes with a fork, 8 to 9 minutes.
Stir mirin, rice vinegar, and remaining 2 tablespoons soy sauce into reserved tea cooking liquid in bowl. Serve fish with mirin-soy sauce spooned over. Top with scallions.
Serve cod in bowls with mirin sauce and sliced scallions.