Toast the hazelnuts in a dry, non-stick pan until golden. Leave to cool a little, then roughly chop or crush using a pestle and mortar. Whisk the mustard with the vinegar and sherry, then slowly pour in the oil, whisking all the time. Season and set aside.
Boil the carrots for 3-4 mins or until just tender, drain and tip back into the pan. Pour over the dressing, stir to coat, cook over a low heat for 1-2 mins, then tip into a serving dish. Scatter over the hazelnuts and serve.