Melt the butter in a large frying pan, add the onions and mushrooms and fry over a moderate heat for ten minutes.

Stir in the tomato puree, mustard and stock, then bring to the boil and simmer for 5 minutes.

Remove from the heat and add the lemon juice and sour cream. Heat through but do not allow to boil. Serve (goes well with fluffy basmati rice).