Preheat the oven to 400°F. Brush a rimmed baking sheet pan with olive oil.
In a large bowl, add the beef, sausage, grated onion, bread crumbs, ½ cup parmesan cheese, garlic, eggs, ricotta cheese, Italian seasoning, salt, and black pepper. Use your hands to gently combine, making sure not to over work the mixture, or your meatballs will get tough. Scoop or roll the meat into 16 equal meatballs and place on the prepared sheet pan. Drizzle a little olive oil over the meatballs. Transfer the pan to the preheated oven and cook the meatballs for 20 minutes. Carefully remove the meatballs from the pan and place in a large bowl with the marinara sauce and toss to coat.
Reduce oven temperature to 350°F. Line a rimmed baking sheet pan with parchment paper.
Transfer the rolls, still attached, to the baking sheet pan. Cut a hole in the top of each roll, about 2/3 of the way down. Brush melted butter over the insides and outsides of the rolls. Bake until golden, about 6 minutes. Place the meatballs with some sauce into the holes. Sprinkle remaining ½ cup Parmesan and Mozzarella cheese over the top. Continue baking until cheese is melted, 12 to 15 minutes.
Remove pan from the oven. Garnish the sliders with fresh parsley and let sit for 5 to 10 minutes to cool. Enjoy!
