Place a slice of bread on top of a piece of plastic wrap on large cutting board. Brush the right edge of the bread with egg wash and place the left edge of another slice of bread over the brushed edge, pressing together firmly. Repeat with the rest of the bread so you have a rectangle of 4 slices across and 3 slices down.
Using a rolling pin, roll out the bread so all of the pieces of bread are sealed together.
Spread mayonnaise over the bread, then arrange the sliced provolone across, starting about ¼ of the way up the bread. Layer on the mortadella, ham, capicola, pepperoni, salami, and peperoncini.
Starting at the bottom, roll the sandwich tightly, then cover snugly with plastic wrap. Chill in the refrigerator for 30 minutes.
Preheat the oven to 350˚F (180˚C). Line a baking sheet with parchment paper.
In a small bowl, melt the butter in the microwave for 1 minute, stirring every 15 seconds, or until fully melted. Mix in the garlic powder and Italian seasoning.
Transfer the sandwich roll to the baking sheet. Brush the outside with the garlic butter and bake for 18-20 minutes or until golden brown.
Let the roll sit for 5 minutes before cutting into thick slices.