To assemble this delectable sandwich, start with splitting the focaccia. Drizzle the inside of each piece with a good quality olive oil. Then, evenly spread the pistachio cream on one side of the focaccia. Next, sprinkle some chopped pistachios or hazelnuts on top of the spread. Follow that up by layering the Stracciatella di bufala and mortadella. Top it with the remaining focaccia slice. Buon Appetito!