To make the garnish, pour the oil into a small frying pan and place over a medium heat for a couple of minutes. (CAUTION: hot oil can be dangerous; do not leave unattended.) Lower the sage leaves gently into the oil with a slotted spoon or tongs, taking care to avoid any splashes or spits of oil.
Fry the sage leaves for 20-30 seconds, or until they look translucent but don’t start to brown. Quickly lift out of the oil with a slotted spoon or tongs and drain on kitchen paper; set aside. They will crisp up even more as they cool.
For the three root mash, peel all the vegetables and cut into 2.5cm/1in chunks. Put the vegetables in a large saucepan and cover with cold water. Bring to the boil over a high heat.
Reduce the temperature slightly and simmer the vegetables for 25-30 minutes, or until very tender.
Drain the vegetables in a colander and then return to the saucepan and mash with the butter, grated nutmeg and lots of salt and pepper to taste. For an extra smooth mash, blend with a stick blender until velvety and smooth.
For the lamb, melt 25g/1oz of the butter in a large, sturdy non-stick frying pan and fry the onions over a low heat for 10 minutes, stirring occasionally. Increase the heat and cook for a further 3-4 minutes, stirring constantly until golden-brown. Tip the onions onto a plate and put to one side.
Return the pan to the heat and add the sunflower oil. Season the lamb chops really well with salt and pepper and fry over a medium-high heat for 2½-3 minutes on each side.
While the lamb is cooking, prepare the kidneys. First give them a good rinse in a colander and then tip onto a board and cut in half right through the middle to give two equal kidney-shaped pieces. Using sharp kitchen scissors, carefully cut out the white central core.
Put the plain flour in a bowl, add a couple of good pinches of salt and a few twists of ground black pepper. Toss the kidneys in the seasoned flour until lightly coated. Shake off any excess flour before frying.
Turn the lamb chops onto their fat side and leave to sizzle for another 2-3 minutes, or until golden-brown. Add the kidneys to the pan with the lamb. Cook the kidneys on one side for three minutes.
Transfer the lamb to a warm plate and leave to rest. Turn the kidneys and cook for a further three minutes on the other side, or until lightly browned and cooked through. If in doubt, turn and cook for an extra minute on each side. Put the kidneys on the plate with the lamb. Drain off any excess fat from the frying pan and place it back onto the heat.
Return the fried onions to the pan and cook for 1-2 minutes, or until hot, stirring with a wooden spoon. Add the Marsala and then quickly stir in the remaining cubes of butter. The sauce will thicken as you stir.
Arrange the lamb chops and kidneys on four warmed plates and spoon over the Marsala onions. Sprinkle with the fried sage leaves and garnish with sprigs of flatleaf parsley if you like. Serve with three root mash and freshly cooked green beans.