Preheat the oven: Set your oven to 350˚F (175˚C).
Prepare your bread: Take the 1 lb. round sourdough bread loaf and slice it. Make parallel cuts about 3/4-inch thick, ensuring you cut down without going all the way through the bottom of the loaf. This creates a fan of spaces for the pesto and cheese.
Create a diamond pattern: Rotate the loaf 45 degrees and slice it again to form a diamond pattern. This will maximize the flavor absorption of the pesto and mozzarella.
Fill the slices: Spread 1 6- to 8-oz. jar of prepared pesto in between the bread slices. Press the slices gently to help the pesto seep into the gaps.
Add mozzarella: Fill the grooves and spaces you created in the bread with 1 8-oz. bag of shredded mozzarella cheese, making sure to distribute it evenly.
Wrap in foil: Take a piece of foil and spray it with nonstick cooking spray to prevent sticking. Wrap it around the loaf securely.
Bake the bread: Place the wrapped loaf onto a baking sheet and bake in the preheated oven until the cheese is melted, approximately 30 minutes.
Broil for finish: After 30 minutes, remove the bread from the foil and place the loaf back on the baking sheet. Increase the oven temperature to broil, and broil the bread for 1 to 2 minutes until the cheese is bubbling and the top is golden brown. Keep a close eye on it to avoid burning.