For the potato fondant, peel the potatoes and cut out cylinders using a circular cutter. Slice one end so it's flat, then fry in the butter until brown (it should be foaming, if not, add more butter!). Add the garlic and thyme, then enough stock to nearly cover the potatoes and transfer to a to 180°C (fan assisted) oven for roughly 40 minutes, or until cooked through.
Score the lamb fat, season and sear on high heat in a heavy bottomed pan with a little oil. When browned all over, remove the lamb and place a good few handfuls of hay into the pan. Allow to smoke a little, then lay the lamb down on the top.
Depending on the size of the lamb, transfer to the oven for 10-16 minutes until it springs. Remove and rest.
To make the peas, fry the pancetta until crisp and add in the sliced shallots and garlic, followed by the peas. Add the stock and cook for 1 minute, before adding the shredded lettuce and the double cream.
Slice the lamb, lay it on top of the peas and potato fondant and serve.