For pesto, in a food processor, combine the basil, Parmesan cheese, pickle, vinegar, mustard and garlic; cover and process until chopped. While processing, gradually add oil in a steady stream; process until blended.
Cut the loaf of bread in half horizontally. Hollow out bottom half, leaving a 3/4-in. shell (discard removed bread or save for another use). Spread two-thirds of the pesto inside shell; layer with beef, cheese, tomato and spinach. Spread remaining pesto over cut side of bread top; place over sandwich. Cut into slices to serve.