Heat the olive oil in a skillet over medium heat. Add the onion and a small pinch of salt. Cook, stirring occasionally, until the onion becomes translucent.
Reduce the heat to low. Add the garlic and continue cooking until the onion is very tender, about 5 to 10 minutes. Remove the skillet from the heat.
In a medium-sized bowl, combine the ground chuck, ground pork, breadcrumbs, milk, eggs, Italian seasoning, 1 teaspoon salt, ground black pepper, red pepper flakes, and 1 tablespoon Parmesan cheese. Use your hands to mix gently. Add the onion and garlic and mix to combine.
Cover and refrigerate for at least 30 minutes.
Preheat the oven to 425°F.
Remove the meat from the refrigerator. Form the mixture into 1 ½–inch meatballs and arrange them on the prepared baking sheet.
Bake until the meatballs are browned and cooked through, about 15 to 20 minutes. Then remove the meatballs from the oven, and reduce the oven temperature to 350°F.
While the meatballs are cooking, pour the pasta sauce into a large pot. Heat the sauce over medium heat. When the meatballs are done, add them to the pasta sauce.
Cut the slider buns in half and place the bottom buns into a casserole dish. Spoon a little sauce and 1 meatball over each bun. Top with a bit of additional sauce and cover with mozzarella.
Place the top buns over the meatballs, brush them with melted butter, and sprinkle with garlic salt and 2 tablespoons Parmesan.
Cover the casserole dish with foil and bake for 10 minutes. Then remove the foil and bake for an additional 10 minutes.