Add the pistachios, olive oil, Parmigiano-Reggiano, basil if using, salt and black pepper to a small food processor. Pulse until thick, spreadable and slightly coarse. Add a little more olive oil only if needed. Taste. Add a few drops of lemon juice only if the paste tastes flat. The paste should be rich and nutty, not loose like a dressing. It needs to sit on the focaccia without running out of the sandwich. That deliberately coarse, thick texture is what lets the paste actually stay put as a real, spreadable layer — anything looser would run straight out of the sandwich the moment it's picked up.

Split each focaccia piece horizontally. If the focaccia is soft or cold, warm it in a dry pan or oven for 3–5 minutes until the edges feel lightly crisp but the inside stays soft. Do not toast the focaccia until hard. This sandwich needs crisp edges and an airy interior, not a crunchy bread shell that squeezes out the filling. That restrained warming, stopping well short of a full toast, protects the bread's own airy interior while still building just enough edge crispness to hold together under a genuinely generous filling.

Spoon the stracciatella into a small bowl. If it is very wet, let it sit in a fine sieve for 5–10 minutes. Season lightly with black pepper. Add only a tiny pinch of salt if needed. Wet stracciatella can soak the focaccia and make the sandwich collapse. It should be creamy and loose, but not watery. That brief draining removes just enough excess liquid to protect the focaccia beneath it, without pushing the cheese's own naturally loose, creamy texture toward something drier and less appealing.

Spread pistachio paste over the bottom half of each focaccia piece. Add a small handful of arugula over the paste. Drizzle with just a little extra-virgin olive oil. The arugula goes under the mortadella so it catches some fat and moisture without making the top slide around. Placing the arugula in this specific position, beneath rather than on top of the mortadella, is what lets it pick up real flavour from the richer layers above while still staying anchored enough that the sandwich doesn't slide apart when eaten.

Fold the mortadella slices loosely and pile them generously over the arugula. Use about 90–120g mortadella per sandwich, depending on how abundant you want the filling. Do not lay mortadella flat. Folding creates height, texture and the abundant layered look from the reference image. That folding technique is specifically what transforms thin, delicate slices into a genuinely voluminous, textured layer — laid flat, the same amount of meat would sit considerably denser and flatter, losing the abundant look this sandwich depends on.

Spoon stracciatella over the mortadella in uneven creamy ribbons. Scatter chopped pistachios over the cheese. Add a very light splash or drizzle of balsamic. Finish with black pepper. Balsamic is for acidity, not sweetness overload. Too much turns the sandwich muddy and fights the pistachio and mortadella. That genuine restraint with the balsamic protects the sandwich's own delicate balance — its job here is purely to cut through the richness of the mortadella, stracciatella, and pistachio paste, not to introduce a competing, dominant sweetness.

Place the top focaccia halves over the filling and press very gently. Cut in half if desired and serve immediately. The sandwich should show airy focaccia, folded pink mortadella, creamy white stracciatella, green pistachio paste, arugula and small dark balsamic streaks. Pressing only gently, rather than firmly, protects the mortadella's own carefully folded volume from being flattened right before the sandwich is even served.