Cut the squash in half lengthwise; scoop out seeds. Place the squash in a microwave-safe dish, cut-side down. Cover and microwave on high until tender, 10-12 minutes.
Meanwhile, in a large skillet, cook the sausage, onion, celery and apple until the sausage is no longer pink, about 5-7 minutes. Drain. Remove from heat and stir in the spinach, cranberries and stuffing mix. In a separate bowl, whisk together egg and milk. Add the egg mixture to the sausage mixture and gently toss to combine.
Turn over the squash. Divide the sausage mixture in the cavity of each half of squash. Place the stuffed squash in the oven and bake for about 10 minutes, or until heated through.
Notes
TheSausage: You can use mild or spicy pork sausage, or try a sage pork sausage (like this one from Jimmy Dean) for even more flavor. Try sweet or spicy Italian sausage, too!
Beef or Turkey: Swap out the sausage and substitute with 1 lb. of ground beef or ground turkey. These meats aren't seasoned like sausage, so you'll likely need to add extra seasoning of your own -- such as salt, pepper, garlic, and any herbs that you like.
Herbs: The stuffing mix includes herbs, but you can also use more of your favorite fresh herbs if you like. Try parsley, sage, thyme or rosemary.
The Stuffing: Instead of the Pepperidge Farm Herb-Seasoned Stuffing mix, try a different brand (such as Stove Top) or a different flavor, such as Cornbread Stuffing Mix.
To stretch the filling, use 3 acorn squash instead of 2. Each cavity will not be stuffed quite as full as you see here, but there should still be enough filling for 6 halves (instead of just 4 servings).
For extra crunch, try adding chopped walnuts or pecans to the filling.