Mushroom caviar

This is a classic from Jane Grigson's The Mushroom Feast and I serve it with toasted walnut brioche.

1. Chop the onions very fine or put them into a food processor and process them to a rough mush. In the same processor bowl, add the fine grater attachment and feed all the mushrooms through, including the stalks, or chop finely by hand.

2. Heat a medium sized frying pan on a moderate heat and add the oil, then cook the onions until almost soft.

3. Season the mushrooms well with salt and pepper. Add them to the pan and cook for six to eight minutes. If the mushrooms exude a lot of juice, turn up the heat so the final result is moist but not wet.

4. Remove from the heat and add the lemon juice. Allow to cool and add the sour cream and chives to taste.

5. Serve topped with extra sour cream and chives and toast on the side.