Step 1: Prepare the Meatballs

1. In a bowl, combine breadcrumbs and milk. Let soak for 3–5 minutes to soften.
2. In a skillet, heat olive oil over medium heat. Sauté onions until translucent (about 3 minutes).
3. In a large mixing bowl, combine beef, pork, egg, parsley, garlic, salt, pepper, oregano, red pepper flakes, Parmesan, the soaked breadcrumbs, and sautéed onions. Mix until just combined — avoid overmixing for tenderness.
4. Shape into 12 equal meatballs and place on a parchment-lined baking tray.

Step 2: Cook the Meatballs
5. Bake at 400°F (200°C) for 15–18 minutes or until golden and fully cooked (internal temperature 165°F).
6. In a saucepan, warm the tomato or marinara sauce and add cooked meatballs. Simmer for 5 minutes to infuse flavor.

Step 3: Assemble the Sliders
7. Slice the slider rolls in half horizontally, keeping the top and bottom layers intact.
8. Place the bottom halves in a baking dish. Lay provolone slices across the buns.
9. Spoon one meatball (with sauce) onto each bun. Sprinkle mozzarella evenly.
10. Place the top halves of the buns back on and gently press down.

Step 4: Brush with Garlic Parmesan Butter
11. In a small bowl, whisk together melted butter, Parmesan, garlic, Italian seasoning, and salt.
12. Brush generously over the tops of the sliders.

Step 5: Bake and Serve
13. Cover loosely with foil and bake at 375°F (190°C) for 10 minutes.
14. Remove foil and bake another 5 minutes, until golden and melty.
15. Let cool slightly before pulling apart. Serve warm and enjoy!