1. Preheat the oven to 350°F (175°C) and grease a 9×13 inch baking dish.
2. Place the chicken breasts in the prepared dish and season with salt and pepper.
3. Top each chicken breast with a slice of Swiss cheese.
4. In a medium bowl, mix together the cream of chicken soup, milk, diced onion, and stuffing mix.
5. Pour the soup mixture over the chicken.
6. Cover the dish with foil and bake for 30 minutes.
7. Remove the foil and bake for an additional 15-20 minutes, or until the chicken is cooked through and the top is golden brown.