Add the meatballs to the slow cooker. Pour over the spaghetti sauce, try to cover all the meatballs. Sprinkle over the parmesan cheese.
Cover and cook on HIGH for 3 hours.
To serve, start assembling the sliders on a cookie sheet. Add a meatball to each slider bun bottom. Sprinkle with parmesan cheese and top each meatball with mozzarella.
Place the sliders in the oven under the broiler (500°).
When the cheese is melted, remove from oven and place the top bun on each slider.