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 350 degrees.
Using a large 9x13 baking dish, spray with nonstick spray, then add your slider bun bottoms. Spoon the meatballs and sauce over the buns, then top with cheese and bun tops. Using a brush, brush melted butter over the top of the buns, then sprinkle parmesan cheese and red pepper flakes. Cover with foil and place into oven. Bake for 15 minutes, then remove foil and bake for 5 more minutes.
Remove and serve immediately.