In a mixing bowl, combine the sliced onions, minced garlic, beefy onion soup mix, dried thyme, and dried rosemary. Stir until the onions are evenly coated with the seasoning to maximize flavor.

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

In a skillet, heat vegetable oil over medium-high heat. Sear the chuck roast for about 4 to 5 minutes on each side until golden-brown.

Transfer the seared roast to the slow cooker, placing it on top of the onion mixture. Add the remaining onion mixture around the roast, then pour the condensed French onion soup over everything.

Top the chuck roast with a few pats of unsalted butter. Cover and set the slow cooker to low heat for 6 to 8 hours or high for 4 to 6 hours.

Carefully remove the roast from the slow cooker and place it on a cutting board. Shred the meat with two forks.

Pour the remaining cooking liquid into a saucepan and make a cornstarch slurry to thicken it.

Return the shredded roast to the slow cooker, mixing it with the gravy. Keep warm until ready to serve, garnishing with parsley if desired.