Preheat oven to 350°F (or preheat grill to medium heat). Lay out a double thickness of standard foil (or a single thickness of heavy-duty foil.) Spray lightly with nonstick cooking spray. Set aside
In a microwave safe bowl, or a heavy-bottomed skillet, heat the butter until melted and mix in the garlic and sliced green onions. Stir with a serving spoon.
Lay the loaf of bread on the cutting board and cut a ½” grid pattern into it stopping about ¼” above the base of the bread so that it stays connected. Gently pry apart the bread and spoon the butter mixture along the seams. Gently wrap the foil up around the top of the loaf and put on a rimmed baking sheet. Bake for 10 minutes.
Remove the pan from the oven, open the foil and sprinkle the mozzarella cheese over the top, gently pushing some of the cheese down into the cut bread. Separate and scatter the pepperoni over the top, shoving some into the sliced bread, too.
Leave the foil open and return the pan to the oven or grill and raise the heat to 425°F or HIGH for another 15 minutes or until the cheese is completely melted and bubbly and lightly browned on top.
Let set for 3 minutes before garnishing with additional chopped scallions, if desired. Serve while still hot or warm.