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Heat oil in a large skillet over medium-high. Season beef with salt and pepper, then sear in batches until browned (3–4 minutes per side). Transfer to slow cooker.
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In the same skillet, add onions and celery. Cook 4 minutes until softened. Add garlic and cook 1 minute more. Stir in tomato paste.
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Pour in Guinness, scraping up browned bits. Simmer 5 minutes to reduce slightly, then transfer mixture to the slow cooker.
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Add carrots, brown sugar, bay leaves, and thyme. Stir to combine.
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Cover and cook on Low 7-8 hours or High 4–5 hours, until beef is tender.
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Skim fat from the top if needed. For thicker stew, stir in a slurry (1 tablespoon cornstarch + ¼ cup hot liquid) and cook 15 minutes on High until thickened.
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Remove bay leaves and serve hot, ideally over mashed potatoes.