Preheat oven to 375°F. Lightly grease a 9×13-inch baking dish with butter or nonstick cooking spray.

Melt the butter in a large skillet over medium heat. Add diced onion and celery and sauté for 5–6 minutes until softened. Add minced garlic and cook for another 30 seconds, until fragrant.

Sprinkle flour over the vegetables. Stir continuously for about 1 minute. Gradually whisk in chicken broth, followed by whole milk. Simmer for 3–5 minutes, stirring frequently, until thickened enough to coat the back of a spoon.

Remove the skillet from heat. Stir in sour cream, dried thyme, garlic powder, and black pepper. Taste and season with salt as needed.

Fold shredded rotisserie chicken and frozen peas and carrots into the creamy sauce. Stir until coated. Spread the mixture into the prepared baking dish.

Prepare the entire 10-ounce bag of herb-seasoned stuffing according to package directions, using the recommended amount of butter and liquid. Chicken broth may replace water if desired. Fluff gently with a fork.

Spoon the prepared stuffing evenly over the chicken filling, completely covering the surface. Do not pack it down. Add chopped fresh parsley if desired.

Cover loosely with aluminum foil and bake at 375°F for 15 minutes. Remove the foil and bake for an additional 25 minutes uncovered. The casserole should be bubbling around the edges and golden on top; confirm the center reaches 165°F.

Let the casserole rest for 5 minutes before serving.