Season the chuck roast generously with salt and pepper on all sides.
(Optional) Heat olive oil in a large skillet over medium-high heat. Sear the roast on all sides until browned, about 3-4 minutes per side.
Place chopped onions, minced garlic, and bay leaves at the bottom of the slow cooker.
Add the seared (or raw) roast on top of the onions.
Arrange the potato and carrot chunks around the roast.
In a bowl, whisk together beef broth, tomato paste, Worcestershire sauce, thyme, and rosemary. Pour this mixture over the meat and vegetables.
Cover and cook on low for 8-10 hours or on high for 4-6 hours, until the meat is tender and easily falls apart with a fork.
Once done, remove the bay leaves and discard.
(Optional) To thicken the gravy, mix cornstarch with 1/4 cup cold water. Stir this slurry into the liquid in the slow cooker and cook on high for an additional 15-20 minutes until the gravy thickens.
Let the roast rest for 10-15 minutes before serving.
Serve hot, with the vegetables and gravy alongside your favorite sides.