In a large sauté pan over medium high heat, brown and crumble the sausage until just barely cooked through. Add 2 tbsp of the butter and the olive oil to the pan and sauté celery and onions until translucent and just beginning to brown, about 10 minutes. Add the apples and sauté for another 5 minutes to soften. Add the cranberries and chestnuts, if using, to the pan. Season well with salt, pepper, and rubbed sage. In a medium saucepan, heat the chicken broth with the remaining 6 tbsp of butter. Place the bread cubes in a very large bowl and add the sausage mixture. Pour buttery broth mixture over the dry stuffing one cup at a time, stirring until absorbed. The mixture should be moist and neither dry or soggy. Season to taste with salt and pepper. Transfer stuffing to the prepared baking dish and bake for approximately 30-35 minutes, until stuffing is lightly browned on top.
