Cut bread in half horizontally, then cut each half crosswise into 6 equal portions. Scoop out the soft center of the bread (freeze for making bread crumbs or discard). Spread cut sides of bread with pesto.
Divide prosciutto, arugula, and mozzarella among six bread bottom portions. Replace tops of bread. Lightly coat both sides of sandwiches with olive oil spray.
Heat a nonstick grill pan or panini press over medium-high heat. Cook three sandwiches at a time in hot pan, weighting sandwiches with a press or heavy skillet. Cook about 3 minutes per side, until cheese is melted and bread is toasted. Cut in half to serve.