Add the ground beef, egg, bread crumbs, onion, garlic, parsley, Italian seasoning, salt, and pepper to a medium mixing bowl and stir well with a wooden spoon or combine with your hands.
Roll meat mixture into 12 evenly sized balls.
Heat a 12 inch oven safe skillet, such as cast iron, over medium heat. Add the olive oil to the skillet and heat until shimmering.
Add the meatballs to the skillet and brown on each side, about 5 minutes total. You may have to work in batches.
Pour the marinara over the meatballs and place the pan into the preheated oven. Bake for 15 minutes or until the meatballs are cooked through.
To make the sliders:
Slice the Hawaiian rolls in half and separate each roll. Place the bottoms of the rolls in a 9x13 baking dish.
Spoon one meatball onto each roll and top with marinara from the pan.
Sprinkle the meatballs liberally with the mozzarella and place the tops of the rolls over the cheese.
Add the butter, garlic, parsley, and onion powder to a small bowl and stir to combine. Drizzle the garlic butter evenly over the tops of the sandwiches.
Cover tightly with foil and bake for 15 minutes.
Serve hot.
Video
Notes
We always have plenty of marinara left in the pan after baking the meatballs and we'll use that for dipping the sliders when serving. If you'd like a crispy, toasted slider, remove the foil from the pan and continue baking for 10 minutes.