Using a serrated knife, carefully slice each croissant horizontally about 3/4 of the way through so the halves remain connected or fully split if you prefer. Place cut sides up on a work surface.

Make mustard mayo

In a small bowl, stir together the Dijon mustard and mayonnaise until smooth. Season with a light pinch of salt (about 1/16 tsp) and a couple grinds of black pepper. Taste and adjust.

If using tomatoes, slice them thinly (about 1/8 inch) and lay slices on paper towel to remove excess moisture. Rinse and dry spinach/arugula.

Assemble sandwiches

Spread about 1/2 teaspoon of softened butter on the cut interior of the bottom half of each croissant. This adds richness and helps flavor balance.

Spread about 1/2 teaspoon of the mustard-mayo mixture on the cut interior of the top half of each croissant.

Layer 2 slices of ham on the bottom half of each croissant, folding if necessary to fit. Top the ham with 1 slice of Swiss cheese on each sandwich.

If using, add a small handful (about 1/4 cup) of spinach or arugula on top of the cheese and 2–3 thin tomato slices. Lightly season the tomato with a pinch of salt and a grind of pepper.

Close the croissant halves together. Insert a toothpick through each sandwich to keep it intact if desired.

Warm and finish (optional)

Preheat a skillet or griddle over medium-low heat. Brush the outside of each croissant lightly with olive oil or melted butter (about 1/4 tsp per side).

Place sandwiches buttered-side down on the skillet. Warm for 1–2 minutes until the bottom is golden and crisp, then flip and warm the other side just until the cheese begins to melt, about 1–2 minutes more. Avoid high heat so the croissant doesn't burn before cheese melts.

Alternatively, preheat oven to 350°F (175°C). Place assembled sandwiches on a baking sheet and heat for 6–8 minutes, or until cheese is melted and croissants are warmed through.

Remove sandwiches from heat, let rest 1 minute, then serve warm.