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Heat enough oil in a medium saucepan to come halfway up the sides. Test if the oil is hot enough by dropping in a ring of shallot; it should sizzle and brown within about 30 secs. Fry the shallots in batches until golden brown. Remove with a slotted spoon, drain on kitchen paper, season with salt and set aside.
Heat a large frying pan until hot. Rub the lamb with a little oil and some seasoning. Cook for 2-3 mins on each side for medium (depending on the thickness of the lamb), then transfer to a plate, loosely cover with foil and leave to rest for 5-10 mins.
Meanwhile, cut away the peel from the grapefruits using a small fruit knife, then cut into segments catching the juices over a large bowl. Add the segments to the bowl along with the dressing ingredients and gently mix. Add the cucumber, chilli, herbs, noodles and any resting juices from the lamb. Toss everything together, making sure all the ingredients are coated in the dressing, then pile onto a serving plate. Cut the lamb into thin slices and place on top of the salad, scatter over the shallots and serve straight away.