Whisk the mirin, miso paste, brown sugar and 1 tablespoon water in a large shallow dish; stir in the sesame seeds. Add the salmon, turning to coat; cover and refrigerate 1 hour or overnight.
Whisk the mayonnaise, wasabi paste and lemon juice in a bowl. Dice the avocado and stir it into the mayonnaise mixture; set aside.
Preheat the broiler. Arrange the rolls, cut-side up, on a foil-lined baking sheet and broil until lightly toasted, about 1 minute. Remove to a plate and brush the cut sides with the avocado mayonnaise.
Toss the watercress with the sesame oil, soy sauce and some salt; divide among the rolls.
Transfer the salmon to the baking sheet and brush with some of the marinade from the baking dish. Broil until the salmon starts browning, about 3 minutes. Flip the salmon and continue broiling until just cooked through, 3 to 5 more minutes. Divide the salmon among the rolls; top with the cucumber slices. Serve warm.
Photograph by Kana Okada
This recipe has been updated to more accurately recognize its origin or to add cultural context. It may differ from what was originally published or broadcast.