Pre-heat oven to 400°
Cut acorn squash in half the short way and scoop out all the guts. You can cut them in half the long way also but I think the short way is prettier.
Brush the squash with olive oil and sprinkle with salt.
Place squash on a baking sheet and bake for 40 minutes or until you can easily pierce with a fork.
While squash is cooking, saute ground turkey for 5 minutes (if you are using cooked turkey skip this step and add turkey in with the veggies).
Scoop ground turkey into a bowl and set aside.
Add onions, celery and cook for 2- 3 minutes, add mushrooms and apples and cook for another 2 minutes (if you are using leftover turkey add it in here).
Add a tablespoon of grated Parmesan cheese and cooked ground turkey and mix thoroughly.
Spoon mixture into squash until very full, sprinkle Parmesan cheese on top and place in oven.
Bake for 15-20 minutes.