In a large skillet, add olive oil and heat over medium heat. Add chopped onion and cook for 2-3 minutes. Add meatballs and marinara sauce; stir to coat. Cover and simmer for about 15 minutes, stirring occasionally. 

Turn off heat and remove lid. Using a spatula, carefully cut the meatballs in half. Stir in with the sauce. Set skillet aside. Preheat oven to 400 degrees.

Using a large 9x13 baking dish, spray with nonstick spray, then add the bottom halves of the slider bun. Spoon the meatballs and sauce over the buns, then top with cheese slices and bun tops.

Using a brush, spread melted butter over the top of the buns, then sprinkle parmesan cheese, red pepper flakes, and Italian seasoning. Cover with foil and place into oven. Bake for 10 minutes, then remove foil and bake for 5 more minutes.

Remove and serve immediately.