Lightly spray the interior of a 4 to 6 quart slow cooker with cooking spray to make cleanup easier later. This step is optional but worthwhile — cranberry sauce can leave a sticky residue on the sides of the insert after a long cook, and a light coating of spray makes the cleanup considerably faster.
Step 2: Mix the Sauce
In a medium bowl, combine the can of whole-berry cranberry sauce and the can of undiluted condensed cream of chicken soup. Stir them together until reasonably well blended — you do not need a perfectly smooth mixture, and the cranberries do not need to be fully broken down. A few streaks and lumps in the sauce at this stage are completely fine and will smooth out during the long cooking time. If you are adding any of the optional ingredients — Dijon mustard, orange juice, onion soup mix, or dressing — stir them into this mixture now.
Step 3: Layer and Cover
Place the raw chicken breasts in a single layer in the bottom of the prepared slow cooker. If the breasts are very large and thick, you can butterfly them or cut them in half to help ensure more even cooking throughout. Pour the cranberry sauce mixture evenly over the chicken, making sure each piece is well coated and the sauce covers the chicken generously. You do not need to add any additional water, chicken broth, or other liquid — the condensed soup provides adequate moisture, and the chicken will release its own juices during cooking, which will thin the sauce naturally to the right consistency.
Step 4: Cook Low and Slow
Place the lid on the slow cooker and set it to LOW heat. Cook for 5 to 6 hours on LOW, or for 2.5 to 3.5 hours on HIGH if you need dinner faster. The LOW and SLOW setting is genuinely preferable here: the longer, gentler cooking produces chicken that is more tender, more thoroughly flavored by the sauce, and far less likely to dry out than chicken cooked quickly on HIGH heat. Chicken breasts are susceptible to becoming dry and stringy when overcooked, and the forgiving nature of LOW heat gives you a wider window of good results. The chicken is done when it reaches an internal temperature of 165 degrees Fahrenheit (74 degrees Celsius) and pulls apart easily with a fork. Resist the temptation to open the lid frequently during cooking — each time the lid is opened, heat escapes and extends the cooking time by 15 to 20 minutes.
