Season the roast on all sides with salt and pepper.
In a large pan, place the butter and vegetable oil over medium high heat. Once hot and the butter is melted, add the seasoned roast and sear until golden brown on all sides. Transfer the roast to a large slow cooker.
Add the onion and garlic to the pan. Cook, stirring occasionally, until golden. Stir in the tomato paste and cook for another minute. Transfer to the slow cooker.
Deglaze the pan with the red wine, scraping up the brown bits from the bottom of the pan, and continue to cook for another 2-3 minutes until reduced by half. Pour into the slow cooker with the roast and onions.
Add the chopped carrots, celery, potatoes, thyme, rosemary, and bay leaves to the slow cooker. Pour in the beef broth.
Cover and set the slow cooker over low heat. Cook until the roast and vegetables are tender, 8-10 hours.
To thicken the sauce: During the last hour of cooking, place the cornstarch in a small bowl. Add about 1/4 cup of the broth from the slow cooker. Whisk until well-combined and pour back into the slow cooker, stirring gently to mix. Continue to cook over low heat for 1 hour. Season as desired with salt and pepper.
Remove the chuck roast from the slow cooker and shred. Discard the thyme stems, rosemary stems, and bay leaves. Sprinkle the roast with parsley and serve warm with the tender vegetables, slightly thickened sauce, and desired sides.