Season the brisket joint with salt and black pepper.
Place sliced onions and bay leaves in the slow cooker, add the beef, and pour in the stout.
Cook the beef on low for 8 to 10 hours.
Heat butter in a pan and cook leeks over low heat for 20 minutes until soft.
Stir in the garlic and thyme, then season with salt and pepper.
Purée the beans, leek mixture, crème fraîche, and parsley in a blender.
Return the mash to the pan to warm through and adjust seasoning.
Serve sliced brisket alongside the bean mash with a spoonful of cooking liquid.