Preheat the broiler to high. Split the baguette or rolls and brush both sides lightly with butter. Broil until lightly toasted, about 2 minutes.
Remove from the broiler and spread both sides with mayonnaise. Toss the scallions, carrot and cucumber in the dressing. Stack the sandwiches with fish, the tossed vegetables, herbs, watercress and chili sauce, if desired.
Thai Red Curry Mahi Mahi Salad
Preheat the broiler to high. Line a broiler pan with foil and lightly oil.
Skim 2 tablespoons cream from the top of the coconut milk (reserve the milk to flavor rice as a side dish, if desired). Fry the coconut cream in a small skillet over medium heat until glossy, about 2 minutes. Stir in the curry paste and cook until fragrant, 2 minutes. Spread the mixture over the fish in the broiler pan; broil until browned and just cooked through, about 15 minutes. Cut the fish into 4 pieces.
Whisk the lime juice, sugar, 2 tablespoons water and fish sauce in a bowl until the sugar dissolves. Stir in the chili sauce and garlic.
Arrange the lettuce, watercress, cucumber, scallions, pepper, herbs, nuts and lime wedges with 4 pieces of fish on a platter; drizzle with the Thai salad dressing.