Split the ciabatta rolls horizontally. Lightly toast the cut sides under a broiler or in a hot skillet until just golden and slightly crisp, about 2–3 minutes per side. Remove and set aside.

Slice mozzarella and apple

Pat the fresh mozzarella dry with paper towels and slice into 1/4-inch rounds. Core the apple and cut into thin slices (about 1/8-inch) so they layer easily on the sandwich. To prevent browning, you may toss apple slices in a little lemon juice from the optional lemon half.

Make balsamic vinaigrette

In a small bowl or jar, combine the balsamic vinegar, Dijon mustard, honey, sea salt, and black pepper. Whisk or shake until smooth. Slowly stream in the extra-virgin olive oil while whisking to emulsify, or close the jar and shake vigorously. Taste and adjust seasoning — if too sharp, add a touch more honey; if too sweet, add a splash more balsamic or a pinch of salt.

Assemble sandwich

On the bottom half of each toasted roll, layer half of the mozzarella slices in an even layer.

Drape 3 slices of prosciutto over the mozzarella on each sandwich. You can fold the prosciutto loosely to create air pockets and texture.

Arrange a thin layer of apple slices over the prosciutto so each bite will have a bit of crisp sweetness.

Toss the arugula in a small bowl with about half of the prepared balsamic vinaigrette (reserve the rest for serving). Use just enough to lightly coat the leaves — about 1/2 tablespoon per sandwich. Pile the dressed arugula atop the apples.

If using, sprinkle the toasted walnut pieces or crushed pistachios over the arugula for crunch and a nutty note.

Drizzle a little of the remaining vinaigrette over the assembled sandwich (about 1/2–1 teaspoon) and close with the top half of the roll.

Finish and serve

Press gently, cut the sandwich in half on the diagonal, and serve immediately. Offer any remaining vinaigrette on the side for extra drizzle. Sandwiches are best eaten the same day they are assembled to keep the bread from getting soggy.