Peel and thick slice the parsnips and onions. Place them in the base of a roasting dish with the beer. Season the joint with salt and freshly-ground black pepper then place it on top of the vegetables. Cover and cook in a preheated hot oven (Mk 7 - 425ºF - 220ºC) for 75-90 minutes depending on your preference. (30 minutes plus 20 minutes per pound for rare).
Remove from the oven and allow the meat to stand in a warm place for 15-20 minutes. Meanwhile, liquidise the parsnips and onions, bring to a simmer and stir well. If necessary, in order to thicken the dip, allow the liquid to boil for a few minutes. Season to taste, then serve in a small pot with each diner plate. Carve the meat into thin slices and heap them on a plate in the middle of the table, along with a basket of hot bread rolls, for people to help themselves.