Cut slice from stem end of squash so squash will stand upright.
Cut out turban end; scoop out seeds.
Lightly salt inside of squash.
Place squash, scooped end down, in shallow baking pan.
Bake at 375° till tender, about 1 hour.
Meanwhile, in skillet combine pork sausage, celery, mushrooms, and onion; cook till vegetables are tender and meat is brown.
Combine egg, sour cream, Parmesan cheese, and the 1/4 teaspoon salt.
Stir into sausage mixture.
Turn squash scooped end up; fill with sausage mixture.
Bake 20 to 25 minutes longer.