Lightly grease a 9x13-inch baking dish with cooking spray or butter.
Slice the slider buns in half horizontally, keeping the bottom halves connected. Place the bottom half of the buns in a greased 9x13-inch baking dish.
Spread half the pizza sauce over the bottom buns. Sprinkle 1 cup of mozzarella cheese evenly on top.
Layer the mushrooms, bell peppers, olives, and onions over the cheese.
Spread the remaining pizza sauce over the vegetables and top with the remaining mozzarella cheese.
Place the top halves of the buns on top.
Mix the melted butter, garlic powder, and Italian seasoning in a small bowl. Brush the butter mixture evenly over the tops of the buns. Sprinkle with grated Parmesan cheese.
Cover the dish with aluminum foil and bake for 10 minutes. Remove the foil and bake for another 5 minutes, or until the cheese is melted and the buns' tops are golden.
Remove from the oven and let them cool for 2-3 minutes.
Slice into individual sliders and serve warm.
Notes
Spread a little extra pizza sauce on the top buns before baking.
Don’t skip the butter topping if you want a subtle garlic-herb flavor.