Season the short ribs with salt and pepper. Heat olive oil in a large Dutch oven over medium-high heat. Brown the ribs on all sides, then remove and set aside.
In the same pot, add the onion and cook until soft. Add garlic and cook for another minute.
Stir in the flour and tomato paste, cooking for about 2 minutes. Slowly add the Guinness and beef stock, scraping any browned bits from the bottom of the pot.
Return the ribs to the pot, add thyme, and bring to a simmer. Cover and transfer to the oven, braising for 2.5 hours or until the meat is tender.
Notes
Serve with mashed potatoes or crusty bread to soak up the rich sauce.