Place the corned beef along with the pickling spices, onion and garlic in a slow cooker/crock-pot with the fat layer on top, pour in the Guinness and cook on low until fork tender, about 8-10 hours.
Set the corned beef aside, strain the solids from the Guinness and juices, place the liquid into a large sauce pan, add the brown sugar, mustard, Worcestershire sauce, salt and pepper, bring to a boil reduce the heat and simmer to reduce by half, about 10 minutes.
Cut the fat layer from the corned beef, glaze with the Guinness glaze and bake in a preheated 400F/200C oven until the glaze starts bubbling, about 10-15 minutes.
