Lightly coat the inside of a 31/2- or 4-quart slow cooker with cooking spray. Layer half of the potatoes and half of the onion in the prepared cooker. In a medium bowl stir together soup, sour cream, Gouda cheese, blue cheese, Parmesan cheese, broth, pepper, and salt. Spoon half of the soup mixture over the potato mixture in cooker, spreading evenly. Top with remaining potatoes and onion. Spoon remaining soup mixture over mixture in cooker, spreading evenly.
Cover and cook on low-heat setting for 6 to 8 hours or on high-heat setting for 3 to 4 hours. Remove crockery liner from cooker, if possible, or turn off cooker. Let potatoes stand, uncovered, for 10 minutes before serving. Sprinkle with parsley.