How to Make It
Preheat oven to 375°F. Melt 2 tablespoons of the butter in a large skillet over medium-high until sizzling. Add onion and celery; cook, stirring occasionally, until softened, about 8 minutes; transfer mixture to a large bowl.
Add 2 tablespoons of the butter to skillet, and melt over medium-high. Add mushrooms and 1/4 teaspoon of the salt; cook, stirring occasionally, until browned, about 7 minutes. Add cream and fresh herbs; bring just to a boil over high. Reduce heat to medium; simmer, stirring often, until cream is reduced and thick enough to coat the back of a spoon, 6 to 8 minutes. Remove from heat; stir in chicken, vinegar, and remaining 3/4 teaspoon salt. Set aside.
Microwave remaining 1/4 cup butter in a small microwavable bowl on HIGH until melted, about 40 seconds, stirring every 20 seconds. Add melted butter, stuffing mix, and stock to onion mixture in bowl; toss until stuffing is moistened. Gently fold in chicken-cream mixture. Spoon mixture into a 13- x 9-inch baking dish coated with cooking spray; cover with aluminum foil. Bake in preheated oven until stuffing is hot, about 20 minutes. Uncover; continue baking until top is lightly browned and crisp, about 20 minutes. Serve.