Combine in a large skillet or casserole with cover:
2 scallions, minced
1 tablespoon minced peeled fresh ginger
1 tablespoon minced garlic
1 tablespoon sugar
1/4 cup soy sauce
1/4 cup mirin
1/4 cup water
1 tablespoon rice vinegar or other mild vinegar
Bring to a boil over medium-high heat. Reduce the heat and lay in a single layer in the liquid:
1 1/2 pounds fish fillets or fish steaks (1 small or 2 large per serving), or 4 small fish (12 to 16 ounces each), rinsed (cod, halibut, mackerel, salmon, butterfish, or any fish that is not too delicate)
Reduce the heat to low and cover the pan. Cook for 5 minutes for 1/2-inch-thick fillets, up to 15 minutes for thick steaks or small whole fish. (To check for doneness, see HOW TO KNOW WHEN FISH IS DONE.) Remove the fish to a platter. If the sauce seems too thin, boil over high heat until slightly thickened.