To Make the Arugula Pesto: Combine the arugula, basil, garlic, pistachios, grated parmesan, and lemon juice in a food processor. Pulse 3-4 times to break up the arugula into fine chunks. On medium low speed, gradually add the olive oil to emulsify. Season with salt and black pepper to taste. Set aside.
To Assemble the Sandwich: Preheat your oven broiler on HIGH. Line a very large, rimmed baking pan with parchment paper.
Slice the French baguette in half lengthwise and place on the prepared pan. Remove as much bread from the insides of the two halves. Drizzle both sides generously with olive oil.
Broil the baguettes inside facing up for 1-2 minutes, until golden and crisp. Remove from the oven.
Flip the top half over to broil on the outside slice. Add the sliced mozzarella to the bottom half of the baguette. Broil another 1-2 minutes, until mozzarella is beginning to brown. Keep a close eye on the top slice, removing after 1 minute or when it begins to get a dark golden brown.
On the bottom half of the baguette, layer with the tomatoes seasoned with salt and black pepper, prosciutto, a generous slather of the arugula pesto, fresh arugula, sliced pepperoncini, and the top half of the baguette.
Slice into 4 sandwiches to be enjoyed!