1. Lightly coat the inside of a 3 1/2- or 4-quart slow cooker with cooking spray. In a large bowl, combine green beans, sweet pepper, Alfredo sauce, onion, water chestnuts, and garlic salt. Spoon bean mixture into the prepared slow cooker.
2. Cover and cook on low-heat setting for 5 to 6 hours or on high-heat setting for 2 1/2 to 3 hours. Serve with a slotted spoon. If desired, sprinkle each serving with crushed croutons.