Add chicken breasts to the bottom of the slow cooker.
Sprinkle chicken breasts with salt and pepper, thyme, parsley, and dry onion soup mix. Lay onions on top of chicken.
Pour condensed french onion soup on top.
Place lid on top of slow cooker and cook on high heat for 3-4 hours or low heat for 5-7 hours (every crock pot is different and cooking time will vary - just be sure that the internal temperature reaches 165 degrees F).
When chicken is finished cooking, preheat oven to broil.
Transfer chicken, onions, and liquid from slow cooker into a large 9x13" baking dish. Top chicken with Swiss cheese slices.
Broil until cheese is starting to turn golden and bubble (about 4-5 minutes).
Serve and enjoy!
Notes
If using chicken thighs, add 30 minutes to 1 hour onto cooking time. Chicken thighs tend to take just a little longer.If you don't want to broil the chicken after cooking it in the slow cooker, you could place the cheese on top of the chicken and place the lid back on while turning the heat to high heat. Let the cheese melt for about 10 minutes, then serve.Store leftovers in the fridge in an airtight container for 4-5 days.