Preheat the oven to 375°F and lightly grease a 9x13-inch baking dish with nonstick cooking spray.

In a skillet over medium heat, cook the ground Italian sausage and diced onion until the sausage is browned and the onion is softened, breaking the meat into crumbles as it cooks.

Add the minced garlic and cook for 30 seconds, then stir in the pizza sauce, Italian seasoning, and red pepper flakes if using. Remove from heat.

Place the bottom halves of the slider rolls in the prepared baking dish in an even layer.

Spoon the sausage mixture evenly over the rolls, then sprinkle on 1 1/2 cups of the mozzarella cheese. Add the pepperoni and olives, if using, then top with the remaining mozzarella cheese.

Place the top halves of the slider rolls over the cheese layer.

In a small bowl, mix the melted butter, garlic powder, and dried parsley. Brush the mixture evenly over the tops of the rolls, then sprinkle with Parmesan cheese.

Cover the dish with foil and bake for 20 minutes. Remove the foil and bake for 10 to 15 minutes more, until the tops are golden and the cheese is melted and bubbly.

Let the casserole rest for 5 minutes before slicing into sliders and serving warm.