Preheat oven to 375 degrees F.
Make the Garlic Sauce: In a medium bowl, microwave the butter on high until melted, about 3 minutes. Whisk in the garlic.
In another medium bowl, whisk together the pizza sauce, tomato juice, garlic, and salt. Set aside.
Slice off part of the rounded side of each bread half to create a flat surface. (This will allow the bread to lay flat in the oven.) Cut the loaves open, lengthwise, keeping them intact like a book. Cut each loaf in half, crosswise, to make 4 sections. Arrange the bread on a baking sheet.
Using a brush, slather the bread with the garlic sauce. Using a ladle or spoon, cover the bread with tomato sauce (be sure to cover to the edges to prevent burning).
Spread about 1/4 of the mushrooms and mozzarella, and 1/2 cup (about 20 slices) of pepperoni across each loaf section and sprinkle with oregano.
Bake until all the cheese is melted and the bread is golden brown, about 8 to10 minutes. Serve immediately.
Recipe courtesy of The Hot Truck, Ithaca, New York