Sprinkle roast with 2 teaspoons of the salt and 1 teaspoon of the pepper. Combine potatoes, carrots, celery, onion, and garlic in a slow cooker. Whisk together broth, flour, and tomato paste in a medium bowl; stir into slow cooker. Add thyme. Place roast on top of vegetables; cook, covered, on LOW until tender, about 8 hours

Transfer roast to a cutting board. Let rest 20 minutes before slicing; then cover with aluminum foil. Using a slotted spoon, remove thyme and transfer vegetables to a serving platter. Sprinkle with 1⁄2 teaspoon of the salt. Cover with aluminum foil.

Whisk together Worcestershire sauce, ketchup, hot sauce, and remaining 1⁄2 teaspoon each of the salt and pepper in a bowl; stir into slow cooker. Increase heat to HIGH, and cook, partially covered, until sauce thickens, 35 to 40 minutes. Serve sliced roast and vegetables with sauce; garnish with herb sprigs, if desired.