Place the meatballs in a large saucepan with a lid. Cover the meatballs with spaghetti sauce (you may not need all the sauce). Cover the saucepan with a lid and bring the sauce to a simmer. Continue to simmer the meatballs in the sauce until they are hot (165 degrees), about 15-20 minutes.
Meanwhile, slice the french bread in half lengthwise. Slice it down the middle from the top of the bread down to the bottom, not from side to side. Turn each half so the cut side is up, then use your finger to tear out a narrow channel of bread.
Divide the slices of cheese between the two halves of bread and tuck them down into the bread. When the meatballs are warm, broil the bread until the edges are brown and toasty and the cheese is melted, just a few minutes.
Spoon meatballs into the bread over the cheese. Sprinkle italian seasoning or parsley and/or parmesan cheese over the top if desired. Slice each french bread half into 3 sandwiches and serve immediately.