Open the canned tuna and drain thoroughly in a fine-mesh sieve or by pressing with a spoon. Transfer drained tuna to a medium mixing bowl and use a fork to flake it into bite-sized pieces.

Prepare Vegetables

Finely dice the celery and mince the red onion. Chop the parsley. Place them in the bowl with the flaked tuna.

Mix Dressing

In a small bowl, whisk together the mayonnaise, Dijon mustard, sweet pickle relish, lemon juice, optional celery seed, salt and black pepper until smooth and well combined. Taste and adjust seasoning (add more salt, pepper or lemon as desired). Note: the recipe lists 3 tsp salt for the whole batch — add gradually and taste, since canned tuna can be salty; you may need less (start with 1 tsp and increase to taste).

Combine

Pour the dressing over the flaked tuna and mixed veggies. Gently fold until the tuna is evenly coated and mixture holds together but remains slightly chunky. Stir in the chopped parsley. Chill for at least 15 minutes (or up to 2 hours) to allow flavors to meld.

Sandwich Assembly & Extras

Prepare Croissants

Preheat a skillet or griddle over medium heat (or oven to 350°F / 175°C). Spread softened butter on cut sides of each croissant. Toast cut-side down in the skillet 2–3 minutes until golden and lightly crisp; alternatively place on a baking sheet and warm in the oven 5–7 minutes.

Prepare Tomato

Slice the tomato into 1/4-inch thick slices. Lightly season both sides with the reserved tomato salt and pepper.

Assemble Sandwiches

Lay two buttered/toasted croissant bottoms on a work surface. Place 2 leaves of lettuce on each bottom half. Divide the chilled tuna salad evenly among the 4 croissants (about 3 oz tuna salad per croissant). Top the tuna with 2–3 tomato slices and optional dill pickle slices if using. Cap with croissant tops.

Finish and Serve

If desired, press sandwiches lightly and cut each in half. Serve immediately. Leftover tuna salad will keep covered in the refrigerator for up to 3 days.