Method
Prepare the bread
Slice the ciabatta roll horizontally. If it's very thick, pull out a little of the soft interior crumb — you want the filling to be the star, not the bread. Drizzle both cut sides lightly with olive oil.
Layer the mortadella
Fold the mortadella loosely rather than laying it flat — you want volume and air, not compression. The fat in mortadella is part of the point; treat it gently.
Add the stracciatella
Spoon the stracciatella in generous dollops over the mortadella. Season it lightly with flaky salt and black pepper. The stracciatella should be cold — the contrast with the room-temperature meat matters.
Finish and serve
Scatter the crushed pistachios over the top, add the lemon zest, and tuck in a few basil leaves. Close the sandwich and press down very gently — just enough to hold it together. Eat immediately.
Why not press it: Mortadella is delicate and the stracciatella is wet. Press it and you'll squeeze out the cheese and flatten the meat into something texturally uninteresting. This sandwich is about lightness. A panino press is for other sandwiches.