First, preheat the oven to 350ºF, and cover a baking sheet with tin foil. Make sure to use enough tin foil to be able to wrap your loaf completely.
In a small bowl, combine melted butter, garlic powder and italian seasoning.
Slice loaf diagonally in both directions into 1-inch sections, creating diamond shapes. Make sure to not cut all the way through the bread, rather leave the bottom crust of the bread unsliced.
Place loaf in the middle of the tin foil covered baking sheet.
Using a pastry brush, brush butter mixture over the loaf, making sure to brush butter in between slices.
Stuff shredded cheese in-between each of the slices, making sure to to get cheese in between each slice. Repeat with pepperoni.
Fold the sides of the tinfoil in towards the sourdough loaf, sealing it shut. Bake at 350ºF for 15 minutes.
Unwrap tin foil from top of loaf and bake uncovered for an additional 8-10 minutes, or until the bread crust is crisp and cheese is completely melted.
Let cool for 5 minutes before tearing pieces off loaf and dipping into warm marinara sauce. Enjoy!