1) Season the brisket with salt and pepper. 2) Heat the oil in a large high sided frying pan over a high heat and brown the brisket on both sides. Remove and set aside. 3) Add the onions to the pan and cook until soft. 4) Return the brisket to the pan and pour over the Guinness. 5) Bring to the boil then turn down to a low simmer and cook covered for 3 hours or until the brisket is tender. 6) Remove the brisket and let it rest for 10 minutes while you make the mash. 7) Melt the butter over a medium heat and cook the leeks, garlic and thyme until soft. Add the beans and stir until warmed through. 8) Pour the bean and leek mixture into a food processor and blitz with the crème fraiche into a puree. Season to taste. 9) Return the mash to the pan for a few minutes if it needs reheating. 10) Serve the brisket sliced with the mash and cooking liquor spooned over.