Spray 4-quart slow cooker with cooking spray. In slow cooker, stir together onions and butter. Cover; cook on Low heat setting 8 hours or until onions are golden.
In large bowl, beat cream cheese with electric mixer on medium speed until smooth. Stir in Parmesan cheese, spinach, artichokes, black pepper and red pepper. Add mixture to onions in slow cooker; stir until blended.
Cover; cook 30 minutes longer or until thoroughly heated. Serve dip warm with pita chips.