Blanch the kale in boiling salted water. Immediately refresh in ice water and set aside for later.
Sweat down the butternut in a pan with some of the butter and cover with the stock. Simmer for 15 minutes until cooked, then blitz and set aside until later. In a pan, seal the lamb on each side until golden brown and place in the oven for 2-3 minutes.
Allow the meat to rest for 3 minutes. Melt some more butter in a separate pan and add the mushrooms. Sweat down lightly then add the kale. Season then take off the heat and place on a dry cloth.
To plate up, put the purée on the bottom followed by the kale and mushrooms. Ensure you slice the lamb at the last possible minute (to prevent the meat from drying out) and place on top and serve.