Cut squash in half, remove seeds, and bake, cut side down, at 350 degrees for 30 minutes.
Meanwhile, saute onions, celery, and carrot in butter or oil until soft and translucent. Add garlic and mushrooms, and continue cooking until they are soft and have released their juices. Add sherry to "deglaze" the pan, and allow to simmer briefly.
Add bread cubes, stir to combine. Add salt/pepper to taste, then enough broth to soften; remove from heat.
Once cooled slightly, add stuffing to interior of squash, mounding as needed, and bake covered another 20-30 minutes. For a crunchier top, remove cover for the last 10 minutes.