Make a slight cut in the acorn squash in the middle where you are going to cut it in half. Microwave on high for 5 minutes. The cut allows it to release pressure that keeps it from possibly exploding
Cut squash in half and scrape the seeds and strings from the cavities. Cut a small slice off squash bottoms if necessary so they will sit level
Place halves cut side down on a rimmed baking sheet and fill with 1/2" water
Roast squash in a 400 degree oven for 30 minutes
While squash is roasting, cook the sausage in a skillet until brown and crumbled. Remove to paper towels to drain
Add onion and celery to skillet and saute over medium heat until softened, about 5-7 minutes
Prepare stuffing according to package directions. Stir in sausage, onions and celery
Remove squash from oven after 30 minutes. Lift off pan with a spatula. Drain water from pan and dry pan. Line pan with foil and place squash back on pan cut side up.
Fill cavities with squash, using your fingers to form it into a large mound
Melt 2 Tbsp of butter and stir in maple syrup. Drizzle slowly over stuffing in squash. Squash can be covered and refrigerated at this point before baking if desired. Remove from refrigerator for 30-45 minutes to bring to room temperature before baking.
Loosely tent squash with foil and return to a 400 degree oven for 30 minutes