Spray 5-quart slow cooker with cooking spray. Arrange onions in slow cooker. Place beef over onions. Sprinkle with taco seasoning mix. Pour broth over beef and onions. Cover; cook on Low heat setting 8 to 9 hours or until beef is tender.
Meanwhile, in food processor, place cilantro and parsley. Cover and process until chopped. Add oil, vinegar, garlic, jalapeño chile, oregano and salt. Cover and process until smooth. Transfer to small bowl; cover and refrigerate the chimichurri sauce at least 1 hour to let flavors blend.
Serve pot roast with cabbage, red onion and chimichurri sauce.