Place the chicken breasts in a single layer in a 6-quart slow cooker.
Make the sauce
In a medium bowl, stir together the cranberry sauce, French dressing, and dry onion soup mix until combined. Pour evenly over the chicken. Sprinkle with rosemary and black pepper if using.
Cook
Cover and cook on low for 5 to 6 hours or on high for 3 to 3 ½ hours, until the chicken reaches 165°F in the thickest part. Check early to keep the meat moist.
Finish and serve
Leave the breasts whole or shred them in the slow cooker with two forks and stir into the sauce. Serve over mashed potatoes, rice, or egg noodles with extra sauce spooned on top.