In a large bowl, combine thinly sliced onions, minced garlic, beefy onion soup mix, dried thyme, and dried rosemary. Mix thoroughly until the onions are well-coated.

Spray the inner pot of your slow cooker with cooking spray. Layer half of the prepared onion mixture at the bottom, followed by one can of condensed French onion soup.

Pat the chuck roast dry with paper towels and season generously with salt and black pepper. In a skillet, heat vegetable oil over medium-high heat and sear the roast for about 5 minutes on each side until browned.

Place the seared roast atop the first layer of onion mixture in the slow cooker. Layer the remaining onion mixture on top of the roast and scatter unsalted butter over it.

Cover the slow cooker and set it on low heat for 6 to 8 hours or high heat for 4 to 6 hours. Avoid lifting the lid during cooking.

Once cooking is complete, remove the roast and simmer the remaining liquid on the stove to thicken with a cornstarch slurry.

Shred the cooked pot roast and mix it back into the gravy, keeping warm until serving.