In a large bowl, combine the thinly sliced onions, minced garlic, beefy onion soup mix, dried thyme, and dried rosemary. Toss everything together until the onions are evenly coated with the spices.

Lightly coat the inside of your slow cooker with cooking spray. Layer half of the onion mixture at the bottom of the slow cooker.

Pat the chuck roast dry and season it with salt and pepper. Heat vegetable oil in a skillet over medium-high heat and sear the roast for about 5 minutes on each side.

Carefully transfer the seared roast to the slow cooker, placing it on top of the onion mixture. Layer the remaining onions over the roast and pour the second can of condensed French onion soup on top.

Cover the slow cooker and set it to low for 6-8 hours or high for 4-6 hours. The roast is ready when it can easily be shredded with a fork.

Once cooking is complete, remove the roast and shred it using two forks. Pour the cooking liquid into a saucepan, and combine the cornstarch with water to create a slurry.

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