How to Make It
In a small saucepan, combine the rice wine vinegar and sugar with 1/4 cup of water and warm over moderate heat, stirring, just until the sugar dissolves. Transfer the pickling liquid to a small bowl and add the cucumber. Refrigerate for 30 minutes, then drain and discard the brine.
Meanwhile, in a food processor or blender, combine the shiro miso, chipotles, honey, lime juice, red wine vinegar, cumin and oregano and puree the sauce until smooth.
Heat a grill pan and brush with the canola oil. Lightly season the fish with salt. Grill over moderate heat, basting the fish liberally with the sauce and turning once, until the fish is golden and cooked through, 10 to 12 minutes.
Arrange the lettuce cups on a platter. Set the fish in the lettuce cups. Top with the pickled cucumber, garnish with sweet potato chips and serve immediately.