Line a 5- to 6-quart slow cooker with a Reynolds(R) Slow Cooker Liner. Open slow cooker liner and place it inside a slow cooker bowl. Fit liner snugly against the bottom and sides of bowl; pull top of liner over rim of bowl.
Combine potatoes, sweet pepper, garlic and black pepper in the slow cooker. 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.
Carefully remove lid to allow steam to escape. Mash potatoes slightly with a potato masher. Stir in whipping cream, Cheddar cheese and 1/2 cup thinly sliced green onions. If desired, top individual servings with additional sliced green onions.
Do not lift or transport liner with food inside. Cool slow cooker completely, spoon out excess liquid, remove liner and toss.