1. Sprinkle beef evenly with soup mix, and place in a 5-quart electric slow cooker coated with cooking spray. Top beef with onion, tomatoes, and garlic. Cover and cook on HIGH 1 hour. Reduce heat to LOW, and cook 6 hours or until tender.
2. Transfer beef to a serving platter; keep warm. Let onion-tomato sauce stand 30 minutes. Skim fat from surface of onion-tomato sauce, and serve over beef.
Note: This brisket can easily be made the day ahead and reheated on the stovetop for a quick meal. Chilling the onion mixture will cause the fat to solidify, allowing you to skim it from the surface with more efficiency.