Remove tenticles and leave the body whole. Saute the onion, celery and garlic in half the olive oil. Chop the tentacles fine and saute them for a few minutes. Add the parsley and bread crumbs, mixing well. Taste and season with salt and pepper.
Stuff the squid, but not too much, and close the opening with a toothpick. They will shrink during the cooking and if stuffed too full they'll split open. Saute them in the bottom of a heavy pot in a little olive oil on all sides.
Pour your favorite marinara sauce over and cook for about 45 minutes, over very low heat, until tender. You can make this ahead and reheat it - it gets even better.