Combine butter and 3/4 cup water in medium-size microwavable bowl. Microwave for 1 minute. Add stuffing mix and cover bowl, venting one side. Microwave an additional 1 1/2 minutes. Stir with fork.
In large bowl, combine celery, tomatoes and ground chicken. Stir until blended. Add stuffing and salt. Stir to combine.
Divide stuffing evenly among peppers. Replace pepper tops, fit into slow cooker (see Note) and add 1 cup hot water. Cover and slow cook for 6 hours on LOW. Remove with tongs and serve.
