Trim any excess fat from beef. Discard fat. Cut beef into serving-size pieces; season with salt and black pepper to taste.
Scrub potatoes. Cut scrubbed potatoes into quarters. Cut carrots and parsnip diagonally into 3/4-inch slices. Slice celery into 1 1/2- to 2- inch pieces.
Place potato, carrot, parsnip, celery, onion and bay leaves in slow cooker. Sprinkle rosemary and thyme over vegetables.
Arrange beef over vegetables in slow cooker. Pour broth over beef.
Cover slow cooker and cook pot roast on LOW about 8 1/2 to 9 hours or until beef is fork-tender.
Remove beef to large serving platter. Arrange vegetables around beef on platter. Remove and discard bay leaves. Serve pot roast with juices from slow cooker or with gravy, if desired.
GRAVY: Ladle the juices from the slow cooker into a 2-cup measuring cup; place in a small saucepan and heat to a boil. For each cup of juice , mix 1/4 cup of cold water and 2 tblsp flour until smooth. Add to the boiling juices, stirring constantly; cook 1 minute or until thickened.