Season the beef chuck roast evenly with salt and black pepper on all sides.
Heat olive oil in a large skillet over medium-high heat. Sear the roast for 3-4 minutes on each side until browned. Remove from heat.
Place the sliced onion, minced garlic, carrots, celery, and sliced mushrooms in the bottom of the slow cooker.
Put the seared roast on top of the vegetables in the slow cooker.
In a medium bowl, whisk together beef broth, red wine, tomato paste, dried thyme, and dried rosemary. Pour this mixture over the roast and vegetables.
Add the bay leaves to the slow cooker.
Cover and cook on low for 8 hours, or until the beef is tender and easily shredded with a fork.
Remove the roast and vegetables from the slow cooker and transfer to a serving platter. Cover with foil to keep warm.
Discard the bay leaves. Pour the cooking liquid into a saucepan and bring to a simmer over medium heat.
In a small bowl, mix cornstarch and cold water until smooth. Gradually whisk the cornstarch slurry into the simmering cooking liquid.
Cook, stirring constantly, until the sauce thickens, about 3-5 minutes.
Serve the pot roast and vegetables with the mushroom gravy drizzled on top.