Preheat oven to 400°F. Line a baking sheet with parchment paper; set aside.

Using a medium bowl, combine the bread crumbs and milk; let soak for 5 minutes.

Meanwhile, add the olive oil to a medium frying pan and place over medium-high heat. Once hot, add the diced onions and cook for 6-7 minutes, or until onions begin to soften. (Note: Leave the skillet out as it will be used again once the meatballs are cooked.)

Transfer onions and soaked bread crumbs into a large mixing bowl. Add all of the remaining meatball ingredients (beef, pork, egg, parsley, garlic, salt, pepper, oregano, red pepper and Parmesan cheese). Mix until just combined. (Tip: Take care not to overmix or the meatball may be tough.)

Shape mixture into (12) golf ball-sized meatballs and place 1” apart on prepared baking sheet.

Bake for 18-22 minutes, or until meatballs are no longer pink in the center.

Reduce oven temperature to 350°F.

Transfer cooked meatballs back into the skillet. Add tomato sauce and stir until well combined. Cook over medium heat, stirring occasionally, for 8-10 minutes, or until sauce has thickened noticeably. Remove skillet from heat.

Cut the dinner rolls in half through the middle. Place bottom half of rolls onto a baking sheet.

Layer the provolone cheese on top of the rolls.

Place the meatballs onto the dinner rolls, putting one meatball on each roll. Spoon extra sauce on top of each meatball. Sprinkle shredded mozzarella cheese evenly on top of meatballs. Cover with the top half of the dinner rolls.

Using a small bowl, whisk all ingredients for the garlic butter together; brush mixture evenly on top of rolls.

Bake at 350°F for 15-20 minutes, or until rolls are golden brown and cheese has melted. (Note: If necessary, place a piece of foil on top of rolls if they are browning too quickly.)

Cut into individual sliders and serve hot.