1-1/8 oz. (1/4 cup) all-purpose flour; more for rolling
Kosher salt and freshly ground black pepper
2 lb. boneless beef chuck, trimmed of excess fat, cut into 1-inch pieces
2 large carrots, cut into 1/4-inch-thick rounds
1 large yellow onion, coarsely chopped
3 large cloves garlic, minced
2 sprigs fresh thyme
1 (12-oz.) bottle Guinness (or other stout)
1 cup lower-salt beef broth
2 large russet potatoes (about 1-1/2 lb.), washed and cut into 1-inch cubes
Nonstick cooking spray
1 sheet frozen puff pastry (about 9 oz.), thawed overnight in the refrigerator
In a large bowl, combine the flour, 2 tsp. salt, and 1 tsp. pepper. Toss the beef in the flour mixture to coat. Transfer the mixture (including excess flour) to a 6-quart slow cooker and then add the carrots, onion, garlic, and thyme. Slowly pour in the Guinness and then stir in the beef broth. Cover and cook on low for 6 to 7 hours or on high for 4 to 5 hours. Add the potatoes and continue cooking until the meat and the potatoes are fork-tender, about 1 hour more.
Position a rack in the center of the oven and heat the oven to 375°F. Coat a large rimmed baking sheet with the cooking spray. On a lightly floured surface, roll the puff pastry sheet into a 10x14-inch rectangle. Put it on the prepared baking sheet and bake until golden-brown, 15 to 18 minutes. Remove from the oven, let cool slightly on a rack, and cut into quarters.
To serve, lay the puff pastry quarters in 4 wide, shallow bowls and spoon the stew over the pastry.