Slow Cooker Guinness Beef Stew

0 Preheat a cast-iron skillet over high heat.
1 Season the beef with ½ teaspoon of the salt and all of the black pepper.
2 Add the oil to the hot skillet. When it shimmers, add the beef in a single layer (work in batches if needed). Brown deeply on all sides, about 4–6 minutes per batch. Don’t rush this—color equals flavor.
3 Transfer the browned beef to the slow cooker.
4 Add the quartered onions, potatoes, carrots, celery, garlic, beef broth, Better Than Bouillon, Guinness, remaining ½ teaspoon salt, tomato paste, thyme, and sugar to the beef.
5 Stir gently to combine. Cover and cook on LOW for 7 hours, until the beef is tender.
9 Melt the butter and whisk in the flour to form a smooth paste.
6 Stir it into the stew.
7 Cover and cook for 1 more hour, or until the stew thickens slightly and coats the back of a spoon.
8 Remove thyme stems before serving. Taste and adjust seasoning if needed.