Combine sliced onion, minced garlic, beefy onion soup mix, thyme, and rosemary in a bowl.

Oil the slow cooker lightly, then add half of the onion mixture along with one can of condensed French onion soup.

Pat the chuck roast dry with paper towels, then season generously with kosher salt and black pepper. Sear it in hot vegetable oil until browned on all sides.

Place the seared roast over the onions in the slow cooker.

Top the roast with the remaining onion mixture, the second can of French onion soup, and the thinly sliced butter.

Cover and cook on low for 6–8 hours or high for 4–6 hours, until the meat is very tender.

Remove the roast and shred it into bite-sized pieces using two forks.

Simmer the cooking liquid in a pot over medium heat, then stir in the cornstarch slurry. Cook until thickened.

Return the shredded meat to the slow cooker and coat it thoroughly with the thickened gravy.

Keep warm until ready to serve, then garnish with freshly chopped parsley.