Heat the oil and butter in a heavy pan and fry the onion for 4-5 minutes over a gentle heat until soft but not browned, stirring frequently.
Add the pumpkin and sliced potatoes, stir well, then cover and sweat over a low heat for about 10 minutes until the vegetables are almost tender, stirring occasionally to prevent them from sticking to the pan.
Stir in the stock, nutmeg, tarragon and seasoning. Bring to the boil and simmer for 10 minutes until tender.
Allow to cool slightly, then pour into a food processor or blender and process until smooth.
Pour back into a clean pan and add the milk.
Heat gently and taste, adding the lemon juice and extra seasoning, if necessary. Se
Serve piping hot.