In a large bowl, combine the panko bread crumbs and milk. Allow to sit for 2-3 minutes.
Add the ground beef, Parmesan cheese, egg, garlic, parsley, salt, pepper, onion powder, and Italian seasoning to the mixing bowl. Mix with your hands until just combined—don’t overwork the mixture.
Roll the mixture into 12 evenly sized meatballs and place them on a parchment-lined baking sheet. Bake the meatballs for 15-18 minutes, or until fully cooked, 160ºF (71.1ºC).
Slider assembly
Remove the Hawaiian rolls from their pack but do not separate them. Slice the rolls in half horizontally with a long serrated knife, keeping the tops and bottoms connected as two large sheets. Arrange the bottom halves of the slider buns in your baking dish (9x13-inch or slightly smaller).
Spread ½ cup of the marinara sauce over the bottom half of the buns. Place a meatball on each bun and evenly spoon the remaining marinara sauce over the top.
Sprinkle shredded mozzarella and Parmesan cheese over each meatball.
Mix melted butter with garlic powder and dried parsley. Place the top halves of the buns on the sliders, then brush the butter mixture over the tops of the buns.
Bake for 10-12 minutes, or until the cheese is melted and the tops are golden brown.
Video
Notes
Make-Ahead Tips: You can make the meatballs up to two days in advance and store them in the refrigerator. They can also be frozen after baking, just cool completely, then freeze in a single layer before transferring to a container or bag. When you're ready to assemble, reheat the meatballs in marinara and proceed with the recipe.Storage: Store leftover sliders in an airtight container in the refrigerator for up to four days. To reheat, wrap in foil and warm in a 350°F oven for 10 to 15 minutes. Fully assembled sliders can also be frozen individually; reheat from frozen or thaw overnight before baking.