Heat the oil in a skillet over medium-high heat. Add the beef in batches, season with salt and pepper, and brown on all sides. Transfer the seared meat to the slow cooker.
Add the onion, carrots, and garlic to the same skillet. Cook for 3 minutes, stirring often, then sprinkle in the flour and stir for 1 minute more.
Pour the Guinness and beef broth into the skillet, scraping up any browned bits. Bring to a quick simmer, then transfer everything to the slow cooker.
Stir in the chopped dark chocolate and tuck the thyme sprigs into the liquid.
Cover and cook on low for 8 hours, until the beef is fork-tender.
Remove the thyme sprigs before serving. Taste and adjust salt if needed.