For the cucumber salsa:
1 cucumber, peeled, seeded, and chopped
1 ½ teaspoons low-sodium soy sauce (or tamari to make it gluten-free)
1 tablespoon rice wine vinegar
2 scallions, chopped
1 tablespoon chopped cilantro
For the sauce:
¼ cup low-sodium soy sauce (or tamari for gluten-free)
¼ cup mirin
¼ cup granulated sugar
2 tablespoons dry white wine
For the fish:
Six (3 to 6 ounce) skinless sea bass fillets (preferably wild striped bass or you can substitute cod or black cod)
½ cup all-purpose flour (optional)
4 tablespoons olive oil
1 tablespoon sesame seeds
Make the cucumber salsa:
In a bowl, combine the cucumber, soy sauce, vinegar, scallions, and cilantro. (You can cover and refrigerate the cucumber salsa for up to 24 hours.)
Make the soy glaze:
In a medium saucepan over low heat, stir together the soy sauce, mirin, sugar, and wine. Heat, stirring frequently, until the sugar is completely dissolved, about 3 minutes. Pour the sauce into a small bowl.
Cook the fish:
If desired, dredge the fish in the flour. Heat a skillet on medium-high until hot. Add the olive oil and heat until hot but not smoking. Cook the fish for 2 1/2 minutes on each side, until browned. Reduce the heat to medium, pour in the sauce mixture, cover, and cook, just until the fish is opaque throughout, 5 to 6 minutes more.
Carefully transfer the fillets to a platter or individual plates and spoon the sauce from the skillet over the top. Sprinkle with the sesame seeds and serve immediately, spooning the cucumber salsa on the fish or passing it on the side.