Melt butter and stir in minced garlic, Italian seasoning, and shredded parmesan to make compound butter.
Spread pizza sauce on one side of each bread slice. Layer mozzarella, 10 pepperoni slices, and Colby Jack on two slices, then top with remaining bread.
Brush the top of each sandwich generously with compound butter.
Heat a cast-iron skillet over medium-low heat. Place sandwiches buttered-side down. Brush the top sides with remaining butter. Cook 4-5 minutes until deep golden brown.
Flip and cook another 4-5 minutes until second side is golden and cheese is fully melted. Remove from pan and rest 3-4 minutes before slicing.