Preheat the oven to 350°F (175°C).
Season the chicken breasts with salt and black pepper on both sides.
Heat olive oil in a large skillet over medium heat. Add the chicken breasts and cook for 4-5 minutes per side until lightly browned but not fully cooked. Remove chicken and set aside.
In the same skillet, add chopped onion, celery, and minced garlic. Cook for 5 minutes until vegetables are softened.
In a medium bowl, combine the condensed cream of chicken soup, chicken broth, sour cream, dried thyme, and dried sage. Stir in the cooked vegetables.
Spread half of the prepared stuffing mix evenly in the bottom of a 9×13 inch casserole dish.
Place the browned chicken breasts on top of the stuffing layer.
Pour the creamy soup and vegetable mixture evenly over the chicken breasts.
Top the casserole with the remaining stuffing mix, spreading it evenly.
Cover the casserole dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the stuffing is golden brown and the chicken is cooked through (internal temperature reaches 165°F or 74°C).
Let the casserole rest for 5 minutes before serving.