Pat beef dry with paper towels and sprinkle with salt and pepper. Heat 2 teaspoons oil in large skillet over medium-high heat. Cook single layer of beef in batches until browned on all sides, about 8 minutes. Transfer to slow cooker insert. Repeat with additional 2 teaspoons oil and remaining beef.
Add remaining 2 teaspoons oil, onions, and 1/4 teaspoon salt to skillet and cook until onions are lightly browned, about 5 minutes. Add broth, 1-1/4 cups Guinness Draught, sugar, thyme, and bittersweet chocolate; bring to boil, using wooden paddle or spoon to scrape up browned bits. Transfer to slow cooker insert.
Add carrots, and potatoes to slow cooker insert. Cover and cook on low until meat is tender, 9 to 10 hours (or cook on high for 6 to 7 hours).
Uncover slow cooker; remove beef, carrots, and potatoes to a large bowl with a slotted spoon. Pour liquid into wire strainer set over large bowl. Discard residual bits in strainer.
Wipe out slow cooker insert; return beef, carrots, and potatoes to slow cooker.
Pour liquid into fat separator. Allow to settle for 5 minutes. Pour liquid into 12-inch skillet. Add remaining 1/4 cup of Guinness and Worcestershire sauce. Bring to boil over medium high heat.
Whisk together Bisto and chicken broth. Whisk into liquid, stirring constantly until liquid thickens. Adjust seasonings.
Pour thickened gravy over beef, carrots, and potatoes in slow cooker. Cover and cook on high for 30 minutes or until thoroughly heated. Serve.
Yield: 6 to 8 servings.