Preheat oven broiler to high.

Place meatballs in a large saucepan and add a full jar (or 3 cups) of marinara sauce. Bring to a simmer over low heat until meatballs and sauce are heated through. (This will take a bit longer if using frozen meatballs, but they don’t need to be defrosted first.)

Meanwhile, combine melted butter and garlic salt in a small bowl. Slice buns along the side and gently open, being careful that the bun stays intact. Place buns on a cookie sheet. Brush open face of the buns with the butter mixture. Coat well (you may not use all of the butter). Place in the oven and broil for 2-3 minutes or just until bread begins to brown. (Please note that the edges of the buns will brown further during the next steps. Remove buns from oven.

Fill each bun with three warmed meatballs, spooning a bit of the sauce over the top.

Top each sub with ¼-½ cup cheese. You can use all mozzarella or a combination of mozzarella and provolone. Place the cookie sheet back under the broiler for 2-3 additional minutes. Watch closely, and remove the subs from the oven as soon as the cheese is melted.