Heat oil in a large skillet over medium-high heat. Add onions, celery and chestnuts; cook, stirring often, until just starting to brown, 3 to 5 minutes. Reduce heat to medium and cook, stirring often, until tender, 3 to 5 minutes more. Add garlic and cook, stirring, for 30 seconds. Add sage and cook, stirring, for 30 seconds. Scrape the mixture on top of the bread. Melt butter in the pan, scraping up any browned bits, then scrape onto the bread. Add broth to taste, sausage and pepper to the bread mixture and stir to combine. Transfer to the prepared baking dish. Spray one side of a piece of foil with cooking spray and cover the stuffing, sprayed-side down.