Sprinkle beef with salt and pepper. In large skillet, heat half of the oil over medium-high heat; brown beef all over, adding more oil if necessary. Transfer to slow cooker.
Discard any fat from pan; add remaining oil. Cook onion, carrots, celery, garlic, marjoram and oregano over medium heat until softened, about 5 minutes. Add mushrooms; cook until tender, about 5 minutes. Scrape into slow cooker.
Stir in stock, scraping up brown bits; boil for 1 minute and pour into slow cooker. Add tomato paste and Worcestershire sauce; cover and cook on low for 5 to 7 hours or until beef is tender.
Transfer meat to cutting board; tent with foil and let stand for 15 minutes before thinly slicing across the grain.
Meanwhile, whisk flour with 1/4 cup (50 mL) water; whisk into slow cooker. Cover and cook on high for about 15 minutes or until thickened. Serve with beef.