I start by placing the cooked (or frozen cooked) meatballs in a large pot. Then I cover them with pasta sauce. If using frozen meatballs, I don’t thaw; just low-medium heat, stir gently (so I don’t break up meatballs), until everything is warmed through.
While sauce and meatballs are heating, I prepare my sandwich rolls. I use ones that are cut so one side remains intact (so it cradles everything). Then I use a slotted spoon to transfer as many meatballs as will fit into each roll (usually 4-5, depending on size), avoiding too much extra sauce.
After filling rolls, I put them on a baking sheet, top with extra sauce if desired, then cover with shredded mozzarella. Bake in a preheated oven at 400°F until the cheese is melted, bubbly, and everything is hot. It takes about 10-15 minutes. If wrapping in foil, I’ll do that before baking for easier transport or softer crust.