Place all sauce ingredients in a saucepan and boil until reduced to approx 3/4 cup. Strain into a small bowl and return to the saucepan.
Place the celeriac and a teaspoon of salt in a saucepan with just enough water to cover. Cook until tender then drain and add butter and cream, mashing or beating with a hand mixer for a smoother result.
Wrap the prosciutto around the lamb and sear quickly on the flat grill, then cook with the lid down over medium, indirect heat for fifteen or twenty minutes.
Rest the meat for 10 minutes before serving, then slice each rump into four or six slices.
Re-heat the sauce and remove from heat before stirring through approx 25 grams of butter just before serving. Season with salt and pepper as required.
Serve a spoonful of celeriac with a couple of slices of lamb and a little of the sauce, with some broccoli, asparagus or beans as a side dish.