Preheat the oven to 375°F (190°C).
Season the chicken breasts evenly with salt, black pepper, garlic powder, and dried thyme.
In a large skillet, heat the olive oil over medium heat. Add the chicken breasts and cook for 4-5 minutes on each side until golden brown but not fully cooked through. Remove chicken from skillet and set aside.
In the same skillet, add the chopped onion and celery. Sauté for 5 minutes until softened.
Melt the butter in the skillet with the vegetables. Whisk in the flour and cook for 1-2 minutes to form a roux.
Gradually whisk in the chicken broth and milk, stirring constantly until the sauce thickens, about 3-4 minutes.
Add dried sage and dried rosemary to the sauce. Stir to combine and remove from heat.
In a 9×13 inch baking dish, spread half of the prepared stuffing evenly on the bottom.
Place the browned chicken breasts on top of the stuffing layer.
Pour the creamy sauce evenly over the chicken and stuffing.
Spread the remaining stuffing evenly over the sauce layer, covering the chicken.
Sprinkle the shredded cheddar cheese evenly over the top of the casserole.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes until the cheese is melted and golden brown.
Let the casserole rest for 5 minutes before serving.