In a shallow dish, whisk together the white miso paste, soy sauce, honey, toasted sesame oil, rice vinegar, minced garlic, and grated ginger until smooth.

Add the salmon fillets to the dish, coating all sides thoroughly with the marinade. Cover and refrigerate for 15 to 20 minutes while you heat the grill and prepare toppings.

Whisk the mayonnaise, sriracha, and fresh lime juice together in a small bowl until completely combined. Set aside in the refrigerator.

Preheat an outdoor grill or indoor cast-iron grill pan to medium-high heat. Lightly oil the grates, remove the salmon from the marinade, and gently wipe away excess glaze to prevent burning.

Grill the salmon skin-side down for 4 to 5 minutes until crispy and lightly charred, then flip carefully and cook for another 3 to 4 minutes until the flesh flakes easily with a fork.

Transfer salmon to a board, remove and discard the skin, and flake the fish into generous, bite-sized chunks.

Arrange chilled lettuce leaves on a platter. Fill each cup with flaked salmon, cucumber matchsticks, a drizzle of spicy mayo, and garnish with sliced scallions and sesame seeds.