Heat olive oil in a skillet over medium-high heat. Sear the beef short ribs on all sides until browned.
Place chopped onion and garlic in the bottom of the slow cooker.
Add the seared ribs on top of the onion mixture.
In a bowl, mix stout beer, beef broth, tomato paste, brown sugar, and Worcestershire sauce.
Pour the sauce over the ribs.
Cover and cook on low for 7–8 hours or high for 4–5 hours.
Once done, remove ribs carefully and spoon off excess fat from the sauce if needed.
Serve hot with your favorite sides.