Preheat the oven to 350˚. Slicing just to the bottom but not through the bottom of the loaf, cut 3/4-inch thick parallel slices. Rotate the bread 45 degrees and repeat the slices, creating a diamond pattern in the loaf.
Spread the pesto in between the bread slices. Fill the grooves with mozzarella. Spray a piece of foil with nonstick cooking spray and wrap it around the loaf. Bake until the cheese is melted, about 30 minutes.
Remove the bread from the foil and return to the sheet tray. Increase the oven to broil. Broil just until the cheese is bubbling and the top is golden brown, 1 to 2 minutes.
