Drain the canned tuna thoroughly in a colander. In a mixing bowl, combine the tuna with mayonnaise, finely chopped cornichons, fresh tarragon, lemon juice, and a touch of Dijon mustard. Mix well and season with black pepper to taste.

Slice large croissants in half lengthwise or create a slight top slice for regular-sized croissants. Lay the croissants on a clean cutting board.

Spread a generous layer of tuna salad on the bottom half of each croissant. Add butter lettuce leaves and slices of tomato, tucking in the lettuce to hold moisture. Top with more tuna salad and press the croissant top down.

Sprinkle a pinch of sea salt over the tomato slices. Serve immediately for the best flavor and texture.