Simmer the Jerusalem artichokes in the milk until softened. Add the cream, sugar and salt and heat to 58°C (136°F). Purée the mixture and then leave to cool in the fridge. Transfer to an ice-cream machine and churn until frozen.
If you don’t have an ice-cream machine, pour into a shallow freezer container. Freeze until semi-frozen, then whisk very thoroughly. Freeze and whisk again, twice more.
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