Season the chicken with the salt and black pepper and put it in a 6-quart slow cooker with the onion and carrots.
Whisk the condensed soup, 1/2 cup broth and Worcestershire and spread it over the chicken.
Cover and cook on low until the thighs are tender, about 4 hours.
Toss the stuffing mix with the remaining broth and melted butter until evenly moistened.
Scatter the frozen peas over the chicken and spoon the stuffing in an even layer on top without stirring.
Cover and cook on high until the stuffing is hot and set and the carrots are tender, 45 to 60 minutes.
Let the casserole stand uncovered for 10 minutes before serving.