Step 1 Zest and juice the whole orange. Place 1 teaspoon zest and all of the juice in a small pot (reserving the additional zest for another use). Add the sugar and sake and cook over medium heat until reduced to one-third cup, 10 to 12 minutes.
Step 2 Reduce the heat to low and add the miso and chili paste, whisking until smooth and thick. Remove from the heat, transfer to a bowl and allow to cool.
Step 3Cut the fish into four equal portions and place it in a shallow glass dish. Pour the orange-miso glaze over the fish. Cover and marinate in the refrigerator overnight or for up to two days. When you're ready to continue, remove the fish from the refrigerator and let it stand while you prepare the cucumbers.
Step 4Combine the cucumbers, vinegar, mirin, onion, sesame oil, salt and pepper in a bowl. Let the mixture stand while you broil the fish.
Step 5Heat the broiler. Set the oven rack to the second shelf. Remove the fish from the marinade and place it on a foil-lined pan. Broil 8 to 10 minutes, until the fish is golden with some charred spots on top and is cooked through (time will vary depending on the thickness of the fish).
Step 6While the fish is cooking, place 1 cup greens on each of 4 plates. Cut the remaining orange half into 8 wedges. Squeeze an orange wedge over the greens on each plate and place a second wedge on each plate.
Step 7To serve, spoon one-fourth of the cucumber mixture onto the greens on each plate and top with a portion of cod. Sprinkle with the thinly sliced shiso leaves.