In a large bowl, combine the sliced yellow onions, minced garlic, beefy onion soup mix, dried thyme, and rosemary. Toss until well coated and fragrant.

Coat the slow cooker with cooking oil spray, then spread half of the onion mixture evenly across the bottom.

Pat the chuck roast dry, season with salt and pepper, then heat vegetable oil in a skillet. Sear the roast for about 5 minutes on each side.

Carefully place the roast atop the onion mixture in the slow cooker. Add remaining onion mixture and pour in the two cans of French onion soup. Top with unsalted butter.

Cover and cook on low for 6-8 hours or high for 4-6 hours until fork-tender.

Remove the roast, shred it into bite-sized pieces. In a saucepan, simmer the cooking liquid. Whisk together cornstarch and water, then stir into the simmering liquid to thicken.

Return shredded meat to the slow cooker, pour thickened gravy over it, and gently stir to combine. Keep on warm setting until serving.