Lop the top off the head of garlic so you can see the tops of the cloves just revealed in the top section. Cut out a square of foil big enough to make a baggy parcel round the garlic.
Put the garlic in the middle and drizzle with olive oil. Make a loose parcel , sealing the edges. Put in a preheated 200 C Gas 6 oven for about an hour until soft.
Cook the peas in boiling salted water according to packet instructions. Drain well then tip into a food processor. Squeeze in the soft cooked cloves of garlic.
Add the butter, parmesan and half of the stock and process to a creamy puree. Pour into a saucepan and add the remaining stock. Check the flavour and add cream to taste.
Heat gently, season to taste and serve hot or chilled.