Add the broccoli, onion, carrots, chicken broth, cream of celery soup, seasoned salt, salt, black pepper, and cayenne pepper to the slow cooker. Stir, place on the lids, and set the slow cooker on high for 4 hours.
After 4 hours use an immersion blender or masher to puree 3/4 of the soup. (If you use a regular blender, blend only 1 cup at a time and use extreme caution.) Add the cheese, turn the slow cooker to low, and place on the lid for 15 minutes.
Stir to melt the cheese and mix it in. Taste the soup and add more salt and pepper as needed.
Serve warm! Soup keeps great in the fridge for a couple of days.