Using a bread knife (or any serrated knife available), slice gently on the diagonal, in one direction, being careful not to slice all the way to the bottom, then criss-crossing in the opposite direction, creating an X pattern.
Spread out a large piece of tin foil, sprayed with cooking spray, and place the bread on it, ensuring the foil is large enough to fit underneath it and close. (You might need two pieces.) Place the tin foil and bread into the baking pan you will be using.
If you haven’t already, prepare your sausage and cut up your pepperoni; set aside.
Melt the butter, and preheat your oven to 350 degrees. Mix the melted butter, olive oil, seasonings, garlic, and mustard together.
Using a small basting brush, baste the seasoned olive oil mixture into every crevice of the bread, making sure every interior surface has been coated. (It is okay to get it on the crust, too.)
Start stuffing the bread, first with sausage, then with pepperoni.
Finally stuff the bread with both cheeses.
Sprinkle the top with a little more Parmesan.
Spray more cooking spray onto the foil before wrapping the foil closed, so the bread won’t stick, then close the foil, forming a pouch.
Place into the oven for 15-20 minutes; remove the foil from the top and place back into the oven for 5-10 minutes to brown.
Remove from foil and place onto a platter to serve.
You can use a bread knife to cut off sections to serve, or have folks pick up one square at a time if they choose.