Spread the butter on one side of each slice of bread and sprinkle with the grated Parmesan, lightly pressing the Parmesan into the butter so it sticks. Lay 2 slices of the bread buttered/cheese-side down in the skillet and then spread 1 tablespoon of the marinara on each. Then top each with 2 slices of provolone, followed by 6 slices of pepperoni, half of the banana peppers and caramelized onions and 2 slices of mozzarella. Spread the remaining marinara on the unbuttered side of the remaining 2 slices of bread, then place one slice of bread on top of each sandwich with the buttered/cheese side facing up.
Cook until golden brown, 3 to 4 minutes per side. Slice and serve.