Spray 3 1/2- to 4-quart slow cooker with cooking spray. In cooker, place beef roast. Remove green tops from onions; chop and refrigerate. Chop white parts of onions; sprinkle over beef.
In small bowl, mix remaining ingredients; pour over beef.
Cover; cook on Low heat setting 6 to 8 hours.
Cut beef into slices; place on serving platter. Serve with sauce; garnish with green onion tops.