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 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.
For the Sliders
Cut the dinner rolls in half through the middle. Place bottoms of the rolls onto a baking sheet.
Layer the slices of cheese on top of the rolls.
Place the meatballs onto the dinner rolls, putting one meatball on each roll. Add a spoonful of 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, make the garlic butter by whisking together the melted butter, Parmesan cheese, garlic, Italian seasonings and kosher salt; brush this mixture evenly on bun tops.
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 aluminum foil on top of rolls if they are browning too quickly.)