In a small saucepan over high heat, bring the broth to a boil. Remove from the heat, add the mushrooms and let stand for 1 hour. Drain the mushrooms, reserving the broth. Roughly chop the mushrooms, transfer to a large bowl and add the focaccia stuffing. Set aside.
Preheat oven to 375 degrees. Butter a small Dutch oven.
In a saute pan over medium heat, melt 3 tbsp of the butter. Add the onion and celery and cook, stirring often, until soft and translucent, about 8 minutes. Add the chestnuts and cook for 2 minutes. Transfer to the bowl with the focaccia mixture.
Return the pan to medium heat and mel the remaining 1 tablespoons butter. Add the sausage and cook, stirring and crumbling with the back of a wooden spoon until the sausage is cooked through, about 10 minutes. Add to the bowl with focaccia mixture and stir in the parsley and sage. In a saucepan over medium-high heat, wanr the reserve broth. Add enough of the broth to the focaccia mixture to make the dressing moist. Season with salt and pepper to taste.
Transfer the dressing to the prepared Dutch oven and bake until crispy and golden on top, 45 minutes to 1 hour, covering the pan with the lid if the dressing gets too dark.