Preheat the oven to 140℃. Get a large frying pan and a casserole dish.
Peel and very finely dice the onions and garlic. You may wish to use a food processor or grater to get this super fine.
Add some olive oil to a large casserole dish and put it over a medium low heat. Fry the onion and garlic gently for 5-6 minutes, just to start softening them. Add the bay, Guinness and prune juice, take off the heat and set aside.
Dice the brisket chunky and season well. Put a large pan over a high heat and add plenty of oil. Fry the beef in batches to brown on all sides. Add the beef to the casserole pot, cover and cook for 3.5 - 4 hours, until the beef is tender.
For best results, allow to sit in the fridge overnight. You are then able to skim off the fat that has collected at the surface. Reheat over a low heat. If necessary thicken the liquid with flour and butter, and adjust seasoning as you need to.
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Notes
Serves really well on it's own, Marco recommends parsnip puree (parsnips simmered in milk and mashed) and fried mushrooms and bacon. Greens and carrots would also serve well, as would baked potatoes or potato mash.