Place the cucumber on a firm, flat, clean surface. Whack it firmly but gently with the flat side of a heavy cleaver or the bottom an 8-inch skillet. Repeat down its length until it is completely smashed. Cut the cucumber into 1–1½-inch pieces and place in a bowl. Toss the cucumbers in the salt and then add the ponzu, sprouts, scallion, and flowers, and set aside.
Whisk together the orange and lime juice with the tahini, tamari, rice wine vinegar, and mirin until smooth. Refrigerate in a sealed container until needed, or up to 5 days.
Preheat the oven to 450°F. Toast the nori in the oven until crisp and fragrant, about 5–10 minutes, and crumble it into small pieces. With a mortar and pestle or a coffee grinder, combine ⅛ cup sesame seeds, salt, and sugar. Combine nori with ground sesame seed mixture and remaining whole sesame seeds. Store in an airtight container.