Combine the meatballs and marinara sauce in a large pot and bring to a gentle simmer over medium heat.

Reduce the heat to low and simmer for 20 minutes, stirring occasionally, until the sauce has thickened enough to coat the back of a spoon and the meatballs are heated through.

Preheat the oven broiler and line a sheet pan with foil.

Slice the slider buns in half as a block and arrange the bottoms on the sheet pan.

Place one meatball on each bottom bun and spoon a little extra sauce over the top.

Sprinkle the shredded mozzarella generously over the meatballs.

Broil for 1 to 2 minutes, watching closely, until the cheese is melted and just starting to brown in spots.

Mix the melted butter with the garlic and parsley, brush it over the top buns, cap the sliders, and serve warm.