If croissants are refrigerated, bring them to room temperature for 10–15 minutes. Using a sharp serrated knife, carefully slice each croissant horizontally about two-thirds of the way through so the top mostly remains attached but the interior is open for filling. Set cut croissants on a work surface.

Make spread

In a small bowl, combine the softened unsalted butter, Dijon mustard, and mayonnaise. Stir until smooth and evenly mixed. Taste and season with about 1/4 teaspoon salt and 1/2 teaspoon black pepper, adjusting to preference.

Assemble sandwiches

Spread about 1/2 tablespoon of the butter-mustard-mayo mixture onto the inside of the bottom half of each croissant and a light smear on the top half. On each bottom half, layer 2 slices of deli ham, then 1 slice of Swiss cheese. If using lettuce and tomato, place 1 leaf of lettuce and 2–3 thin tomato slices atop the cheese. Finish by closing the croissant over the filling.

Season

Lightly sprinkle a pinch of salt and a small grind of black pepper over the tomato (if used).

For Warm (Toasted) Finish

Pan-toast (stovetop) option

Heat a nonstick or cast-iron skillet over medium-low heat. Brush the outer tops of croissants with a thin film of melted butter or olive oil. Place croissants, buttered-side down, in the skillet and press gently with a spatula or weighted press. Cook 2–3 minutes until golden and the cheese begins to melt, then flip and warm the other side 1–2 minutes. Remove and serve immediately.

Oven option

Preheat oven to 350°F (175°C). Wrap assembled croissants loosely in foil or place on a baking sheet, brush tops lightly with butter or oil, and warm in the oven 6–8 minutes until heated through and cheese is melted. Unwrap and serve hot.

Serving

Serve sandwiches whole or halved on the bias. Offer extra Dijon or mayo on the side if desired. Leftover assembled (but not heated) croissants can be refrigerated up to 24 hours; for best texture, reheat in a 350°F (175°C) oven for 6–8 minutes before serving.