In a small bowl, whisk together tomato sauce, beef broth, minced garlic, thyme, worcestershire sauce and white wine vinegar.
Trim any excess fat off of your chuck roast. Salt and pepper all sides of the roast.
In the bottom of your slow cooker, add half of the sliced red onions. Pour half the sauce over the onions. Place chuck roast on top of the onions then evenly sprinkle remaining onions on top. Pour remaining sauce on top.
Cover and cook on high for 4 hours or on low for 7 hours. When ready, roast will be fork tender.
Spread mayonnaise (or butter) on rolls and place cut side down on a hot skillet until nice and golden. Place rolls open on a baking sheet and on one side of the roll, pile up the beef. Top beef with Swiss cheese and place under the broiler for about a minute until cheese is melted.
Serve with "au jus" juices from the slow cooker in a small dipping bowl with your sandwich. Enjoy!