Prepare the sauce by whisking together the tomato sauce, beef broth, minced garlic, thyme, Worcestershire sauce, and white wine vinegar in a small bowl until fully blended.

Trim excess fat from the chuck roast and season all sides with salt and black pepper.

Layer half of the sliced onions in the slow cooker, pour half of the sauce over them, place the roast on top, sprinkle with remaining onions, and pour the rest of the sauce over.

Cover and cook on low for about 7 hours or high for 4 hours until fork-tender.

Remove the roast and transfer sauce to a saucepan; whisk in cornstarch or flour over medium heat until thickened.

Slice or shred the roast, serve over mashed potatoes with gravy, and garnish with parsley.