Pat roast dry; season all sides with salt and pepper.
Heat oil in a skillet over medium-high. Sear beef until browned.
Scatter onion and garlic in the slow cooker.
Whisk vinegar, broth, brown sugar, Worcestershire, tomato paste, Italian seasoning, and red pepper flakes.
Pour sauce over onions; add seared beef. Scrape skillet drippings into cooker.
Cover and cook on Low 8–9 hours, or High 4–5 hours.
Transfer beef to a board. Skim fat from the cooking liquid.
Stir cornstarch into cold water. Whisk slurry into the slow cooker until slightly thickened.
Shred beef with forks. Return to sauce, toss, and garnish with parsley.