In a large bowl, combine ground beef, 1/4 cup parmesan, parsley, oregano, basil, fennel, 1/4 teaspoon crushed red pepper flakes, and a large pinch of salt and fresh ground pepper. Divide into twelve even portions and then shape into patties about 1/2-inch thick. Place on a plate and set aside.
Preheat oven to 350°F. Set a grill pan over moderate heat.
Once the grill is hot, grease with cooking spray or brush with oil. Grill the patties, a few at a time, until charred and cooked through, about 4 to 5 minutes. Transfer to a baking sheet and continue grilling the rest.
In a small bowl, stir together melted butter, garlic, Italian seasoning, a pinch of salt and pepper and remaining 1/4 teaspoon crushed red pepper flakes. Place slider buns on a baking sheet and brush liberally with the garlic butter. Sprinkle the tops with plenty of the remaining 1/4 cup parmesan cheese. Bake until lightly golden brown, about 10 minutes.
Top the grilled patties with a slice of cheese and three pepperoni slices each. Bake until the cheese has melted slightly, about 10 minutes.
To assemble, split open the buns and spoon about 1 tablespoon of pizza sauce onto the bottom bun. Place a patty on each and top with another tablespoon of sauce. Place top bun on and they're ready to serve. Enjoy!