Cut the swede, carrots and potatoes into small chunks. Put them into a pan of cold water, salted at the rate of 1 teaspoon per 600ml/1 pint, bring to the boil and cook for 20 minutes until tender. Drain well and leave until the steam has died down.
Transfer the vegetables to a food processor with the butter and blend briefly to make a smooth purée. Return to the pan and leave to cook over a gentle heat, stirring now and then, for a few minutes until the mixture has dried out a little.
Stir in the chopped rocket and season to taste with salt and plenty of freshly ground black pepper.
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