Whisk together the tomato sauce, beef broth, minced garlic, thyme, Worcestershire sauce, and white grape juice vinegar in a small bowl.
Trim any excess fat from the chuck roast and season all sides with salt and black pepper.
In the slow cooker, add half of the sliced onions, pour half of the sauce over, place the seasoned chuck roast on top, sprinkle remaining onions, and pour the rest of the sauce over.
Cover the slow cooker and set it on high for 4 hours or low for 7 hours until the roast is fork-tender.
Make gravy using the juices from the slow cooker by whisking in cornstarch or flour over low heat until thickened.
Serve the pot roast with mashed potatoes or your favorite side, drizzling gravy over everything.