In a 4- to 6-quart slow cooker, combine potatoes, sweet pepper, garlic, and black pepper. Pour broth over all.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
Mash potatoes slightly with a potato masher. Stir in whipping cream, cheddar cheese, and the 1/2 cup thinly sliced green onions. If desired, top individual servings with additional sliced green onions. Makes 8 servings.