Adapted from the Stuffed Winter Squash recipe from Alana Chernila's cookbook, Homemade Kitchen: Recipes for Cooking with Pleasure.
Preheat oven to 375°F and line a baking sheet with foil. Rub the flesh of the squash halves with olive oil and sprinkle with salt. Lay each cut-side down on the sheet and bake approximately 30-40 minutes (adjust to 20-30 minutes if using smaller or thinner-skinned squash, like delicata). You will know the squash is done when the flesh can be easily pierced with a fork. Remove from the oven and raise the temperature to 425°F.
Once the squash has been roasting a few minutes, start the stuffing. Heat a large skillet over medium heat. Add the sausage and fry until browned (5-10 minutes). Remove with a slotted spoon and set aside, leaving the drippings in the pan.
Melt the half tablespoon of butter in the pan and add the carrots and mushrooms. Cook until the vegetables are softened (approximately 8-10 minutes), stirring occasionally. Mix in the apple and add salt and pepper to taste, cooking 1 minute. Add back in the sausage, then the spinach, sage, and rice and cook one more minute while tossing the ingredients to combine. Remove the pan from heat.
To assemble, turn the squash over gently so that the flesh side is up. You can either use the same baking sheet for the next step, or transfer the squash to more presentable casserole dish if you plan to serve family style at the table - Whichever you prefer. Fill the cavities of the squash with the rice stuffing, heaping it as high as possible. Sprinkle the cheese over the tops of each stuffed squash, and bake for 10 minutes.
You can make this recipe in advance! All of the chopping can take some time, so if you’d like to prepare the dish earlier in the day or the day before you plan to serve to guests (or family), follow the instructions through step #3. This will leave you with pre-roasted squash halves and the filling completely prepared. For best results, store the filling and the squash halves in separate containers in the fridge, and be sure the squash is able to sit flat and with enough room to “breathe” until use - in other words, in order to keep them presentation ready don’t squish the squash together. When you are ready to serve, simply place squash halves in a baking dish, fill each with the filling, and sprinkle them with cheese (as described in step #4). You may need to add an additional 5-10 minutes to the baking time depending on the size of your squash in order to take off the chill from the refrigerator (be sure to test the center of one to make sure it’s heated through!). Your guests will never know the difference.