method
1. Peel and dice the potatoes; place in a large, heavy-bottomed saucepan with the peas, shallot, basil leaves and stock. Bring to the boil, cover and simmer over a low heat for about 30 minutes or until the potatoes are very tender.
2. Drain off and reserve the liquid remaining in the saucepan; puree the cooked mixture by putting it through the vegetable mill with a fine-gauge disc fitted, or rub through a sieve.
3. Return the puree to the saucepan; stir in 2 - 3 tbsp of the reserved liquid and heat to just below boiling point over a lot heat. Beat the egg yolk into the puree for a few seconds only, just enough to blend well, using a balloon whisk.
4. Remove from the heat, season to taste with a little salt, some freshly ground pepper and a pinch of nutmeg; stir in the garlic and then the grated cheese. Keep beating as you add the olive oil a very little at a time.
Serve at once.
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