Place the entire roast in the same pan you used for the onions.
Sear each side of the roast until it turns golden brown.
Place the roast on top of the onions in the slow cooker.
Cover the roast with the remaining onions.
Pour the beef stock and Guinness on top of the roast and add the remaining ingredients. Set the slow cooker to high and let cook for at least 5 hours. More is better if you have the time.
Once the meat is fully cooked through, taste it. Add salt, pepper and other seasonings if needed. Serve on top of mashed potatoes with a side of veggies.