Pat the chuck dry, season with salt and pepper, and brown it in oil in a large skillet for 4 to 5 minutes per side.
Put the onion, carrots, parsnips, turnips and garlic in a 6-quart slow cooker. Set the beef on top.
Add 1 1/2 cups broth and the Worcestershire to the skillet, scrape up the browned bits and pour around the roast. Add thyme and bay leaves.
Cover and cook on low for 8 to 9 hours, until the beef and roots are tender. Move the beef and vegetables to a platter and discard the bay leaves.
Whisk the remaining 1/2 cup cool broth with the flour, stir it into the liquid and cook on high for 10 to 15 minutes. Serve the sliced or pulled beef with the roots and gravy.