Season the roast with salt and pepper. Brown it in 1 tablespoon oil in a hot skillet for 4 minutes per side and transfer it to a 6-quart slow cooker.
Add the remaining oil and mushrooms to the skillet. Cook for 7 to 9 minutes until browned; add the onion and cook 4 minutes more. Stir in the garlic and tomato paste for 1 minute.
Add the wine, broth, Worcestershire and thyme. Simmer for 2 minutes while scraping the skillet, then pour the mixture around the roast.
Cover and cook on low for 8 hours until the beef is fork-tender. Transfer the meat to a board and skim the sauce.
Mix cornstarch and water, stir it into the mushroom sauce and cook on high for 10 minutes. Slice or pull the roast and spoon the gravy over it.