In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, parsley, minced garlic, egg, salt, and pepper.
Use clean hands to gently mix until just combined, being careful not to overwork the meat which can make meatballs tough. Let the mixture rest in the refrigerator for 15 minutes to allow flavors to meld and make forming easier.
With slightly dampened hands, portion the meat mixture into 12 equal balls, about 1.5 inches in diameter.
Heat olive oil in a large skillet over medium heat until shimmering. Cook meatballs, turning occasionally for even browning, about 8-10 minutes total until they are cooked through with an internal temperature of 160°F.
Once the meatballs are fully cooked, reduce heat to low. Place a slice of fresh mozzarella on top of each meatball directly in the skillet.
Cover with a lid and allow the cheese to melt completely, about 1-2 minutes. Remove from heat once the cheese has melted to prevent burning.
While the cheese is melting, mix minced garlic with softened butter and spread on the inside of each slider bun.
Toast buns in a separate pan or under the broiler until golden, about 1-2 minutes. Keep warm until ready to assemble.
Place a cheese-topped meatball on the bottom half of each toasted bun. Spoon warm marinara sauce over each meatball, being careful not to add too much to avoid soggy buns.
Garnish with fresh chopped basil. Cap with the top bun and secure with a toothpick if desired. Serve immediately.