Spray 5- to 6-quart slow cooker with cooking spray. In Dutch oven, cook beef and 3 cups of the onions over medium-high heat, stirring frequently, until beef is thoroughly cooked; drain. Stir in salt, cinnamon, cocoa, cumin and water. Simmer 2 minutes. In slow cooker, mix beef mixture, tomato puree and brown sugar until blended.