Toss the beef with flour, salt, and black pepper until evenly coated; the pieces should look dry-dusted. Heat vegetable oil in a Dutch oven over medium-high until shimmering.

Brown the beef in hot oil, turning as needed, until well-browned on multiple sides; transfer to the slow cooker as it finishes. Keep the oil hot for better color, aiming for a deep, crusty exterior.

In the same pan, cook the onion, garlic, and tomato paste for exactly 3 minutes. Stir until fragrant and slightly darkened, with the tomato paste clinging to the pan.

Pour in the Guinness stout and scrape the bottom of the pan to loosen the browned bits; simmer for 2 minutes. You should see steam and a glossy dark sauce form in the pan.

Pour the stout mixture into the slow cooker over the browned beef. Spread it so the beef is surrounded for even braising.

Add beef broth, Worcestershire sauce, brown sugar, dried thyme, sliced carrots, and halved baby potatoes to the slow cooker. Give everything a gentle stir so vegetables are distributed.

Cover and cook on LOW for 8 hours until the beef is very tender and the potatoes are soft. Look for active steam and a thickened, dark-brown stew consistency around the edges.

Season to taste with additional salt and black pepper as needed. The stew should taste balanced and rich without sounding overly bitter.

Garnish with chopped fresh parsley right before serving. Add it at the end for a bright green color and fresh aroma on top.