Place ¼ cup all-purpose flour, 2 teaspoons dried thyme, 1 ½ teaspoons kosher salt, 1 teaspoon black pepper, and 1 teaspoon garlic powder in a 6-quart or larger slow cooker and stir to combine.
Dice 1 medium yellow onion (about 1 ½ cups). Peel and cut 4 medium carrots crosswise into 1-inch-thick rounds (about 2 ½ cups). Cut 2 medium celery stalks crosswise into ½-inch-thick pieces (about ¾ cup). Cut 1 pound Yukon gold or red potatoes into 1-inch pieces.
Trim off any large pieces of surface fat from 2 ¼ pounds boneless beef chuck roast, then cut into 1 to 1 ½-inch pieces. Transfer to the slow cooker and toss to coat in the flour mixture. Add the vegetables and 2 dried bay leaves.
Place 1 ½ cups beef broth, 1 tablespoon tomato paste, and 1 tablespoon Worcestershire sauce in a liquid measuring cup or medium bowl and whisk to combine. Pour over the beef and vegetables and stir to combine.
Cover and cook until the beef is very tender, 6 to 8 hours on the LOW setting or 4 hours on the HIGH setting. Taste and season with more kosher salt and black pepper as needed. Serve topped with chopped fresh parsley leaves and more black pepper if desired.
