How to Make It
Place potatoes, carrots, parsnips and bay leaves into a 6-quart slow cooker. Season with salt and pepper.
Mix mustard, thyme, 1 Tbsp. flour and 1/2 tsp. each salt and pepper in a small bowl; rub mixture all over roast. Heat 1 Tbsp. oil in a large skillet over high heat and sear meat on all sides until browned, 2 to 3 minutes per side. Add to slow cooker, on top of vegetables.
Wipe out skillet and heat remaining oil over medium heat. Add onions; cook until translucent and beginning to brown, about 8 minutes. Stir in remaining flour and cook, stirring, for 1 minute. Add tomato paste and cook for 1 minute more. Slowly stir in beef broth and Worcestershire sauce; bring to a simmer. Cook until thickened, about 2 minutes.
Pour onion mixture into slow cooker. Cover and cook on low until meat is tender, about 8 hours. Slice meat and place on a platter; transfer vegetables using a slotted spoon. Discard bay leaves. Garnish with parsley, if desired. Serve immediately.