Wipe mushrooms clean with a wet paper or cloth towel. Saute the mushrooms with a little oil and save the juice. Saute the onion separately.
In a food processor, blend all the ingredients except the mushrooms. Fold in the mushrooms without blending. Place the mixture in an oiled cake pan. Bake at 180C/350F until it cracks and turns light brown in colour (35 minutes approximately).
Allow to cool and then chill. Serve cold in slices.