Preheat oven to 400F. Cut butternut squash in half length-wise, leaving the skin on. You can cup off the stem to make it easier to cut in half. Seed squash and puncture sides with a fork.
Whisk together maple syrup, olive oil, minced garlic, salt, and pepper.
Brush the flesh of the squash with the maple marinade mixture.
On a lined baking sheet, pre-bake the squash (before stuffing) for 35 minutes, until the flesh is soft and easily pierced with a fork but not falling apart or mushy. If the neck of the squash is still a bit raw, cook an additional 5 min.
While the squash is pre-baking, prepare your filling: Combine olive oil, diced onion, diced celery, salt, chili flakes, poultry seasoning, and black pepper in a large sauté pan over medium-high heat. Cook until glossy and softened, about 8 minutes.
Then, add the ground sausage to the pan and break it up with a spatula. Cook the sausage until completely browned.
Stir in fresh spinach.
Cook until spinach is wilted, a few minutes, and then add ½ cup of shredded cheddar cheese, stirring to combine.
Stuff each cavity of butternut squash with ¼ of the filling. Top with the remaining ½ cup shredded cheese. These should be overly full!
Once stuffed, bake for an additional 15-20 min until cheese on top is slightly browned and melted.