We found that canned tomato sauce works well here. Depending on the size of the loaf of bread, you may need to trim it. A rasp-style grater makes quick work of turning the garlic into a paste.
Adjust oven racks to upper-middle and lower-middle positions and heat oven to 450 degrees. Combine oil, garlic, ¼ teaspoon salt, and ¼ teaspoon pepper in bowl. Brush cut sides of bread with two-thirds of garlic oil, press bread halves back together, and wrap in aluminum foil. Toss broccoli with remaining garlic mixture.
Pat chicken dry with paper towels and season with salt and pepper. Place chicken in center of rimmed baking sheet and top evenly with tomato sauce. Combine mozzarella and Parmesan in separate bowl, then sprinkle over chicken. Scatter broccoli on sheet around chicken.
Place foil-wrapped bread on lower rack, and place sheet with chicken and broccoli on upper rack. Bake until chicken registers 160 degrees and broccoli is tender, 20 to 25 minutes, rotating sheet halfway through baking.
Meanwhile, microwave pepperoni on paper towel–lined plate to render fat, about 30 seconds.
Remove sheet from oven. Sprinkle pepperoni over chicken, tent loosely with foil, and let rest for 5 minutes. Slice garlic bread. Before serving, sprinkle basil over chicken.
