For the tomato sauce: Heat the olive oil in a large, deep skillet over medium-high heat. Add the onions and cook until translucent, about 5 minutes. Add the garlic and pepper flakes and cook until fragrant, 1 to 2 minutes. Add the tomato paste, cook for 1 minute. Stir in the tomatoes, basil, thyme, 1 cup water, and some salt and pepper. Reduce the heat to medium-low and simmer for 15 to 20 minutes. Keep warm.
For the meatballs: Soak the panko in the milk for 10 minutes. In a large mixing bowl, combine the ground beef and pork, Parmesan, parsley, garlic, some salt and pepper and the soaked panko. Mix well to combine; do not overwork. Form into eight 2-inch balls.
Set a large saute pan over medium-high heat. Add the olive oil. Sear the meatballs on all sides until golden brown, 6 to 7 minutes.
Remove the thyme sprigs from the tomato sauce; add the meatballs to the sauce. Spoon some sauce over the meatballs, then top them with slices of mozzarella. Transfer to the oven and bake until the cheese is bubbly and the meatballs are cooked through, 20 to 25 minutes.
To assemble the sliders, load one meatball onto each bun. Top with a basil leaf and secure the bun top with a bamboo skewer. Serve on a large platter with extra sauce on the side.