Rinse the lettuce leaves and pat them dry with paper towels or a clean kitchen towel. Slice the tomato into 6 thin rounds (about 1/4-inch thick). Season the tomato slices lightly with 1/2 teaspoon of the salt and 1/4 teaspoon of the black pepper; set aside.

Split croissants

Using a serrated knife, gently slice each croissant horizontally about two-thirds of the way through so the top remains attached (or slice completely if preferred). Place croissants cut-side up on a work surface.

Toast (optional)

If you like warm, slightly crisp croissants, heat a large skillet over medium-low. Add 1 tablespoon (tbs) of butter and let it melt and foam, then place croissants cut-side down for 1 to 1 1/2 minutes until lightly golden and crisp. Alternatively, brush cut sides with melted butter and broil for 30–60 seconds, watching carefully. Remove and cool briefly.

Mix spreads

In a small bowl, stir together the mayonnaise (6 tsp) and Dijon mustard (3 tsp) until smooth. Taste and adjust ratio if desired.

Assemble sandwiches

Spread about 1 teaspoon of the mayo-Dijon mixture on the bottom half of each croissant and about 1 teaspoon on each top half (use remaining if you prefer more). On each bottom half layer 2 slices of deli turkey (fold if needed to fit), then top with one slice of Swiss cheese. Add one lettuce leaf and one seasoned tomato slice on top of the cheese. Drizzle each tomato with a light brush of olive oil (about 1/6 tsp per sandwich) and finish with a light pinch of remaining salt and pepper (divide remaining 1/2 tsp salt and 1/4 tsp pepper across sandwiches). Close the croissant tops.

Warm to melt cheese (optional)

If you prefer the cheese slightly melted, place assembled croissants in a preheated oven at 325°F (160°C) for 4–6 minutes, or place in a skillet with a lid over low heat for 1–2 minutes until cheese softens. Do not overheat or the croissant will become soggy.

Serve

Cut each croissant sandwich in half on the diagonal if desired and serve immediately. Provide napkins — croissants are buttery and flaky.