Prepare the pistachio pesto by placing the pistachios, basil, garlic, Pecorino Romano, and lemon juice in a food processor.
Pulse the ingredients while slowly drizzling in the extra virgin olive oil until a slightly coarse paste forms. Season with salt and pepper.
Lightly toast the cut sides of the focaccia bread under a broiler for 1 to 2 minutes until golden brown.
Spread a thick layer of the pistachio pesto on the bottom half of the toasted focaccia.
Layer the mortadella slices over the pesto, folding each slice to create height and texture.
Place the burrata balls on top of the mortadella and gently break them open with a knife, spreading the creamy interior across the sandwich.
Top the burrata with a handful of fresh baby arugula.
Place the top half of the focaccia over the fillings, press down gently to integrate the layers, and slice into two portions.