Separate the mushroom stalks from the caps, Rinse and chop in a food processor.
Put the chopped stalks in a bowl with the butter and lemon juice.
Microwave, uncovered, on HIGH for 2 minutes.
Wipe the mushroom caps with a damp cloth or kitchen paper and put face down in a circle in a small casserole.
Season with salt and pepper.
Cover and microwave on HIGH for 3 minutes.
Cut the crab sticks into pieces with kitchen scissors.
Mix with the chopped parsley, half the sour cream (creme fraiche), the chopped mushroom mixture, and salt and pepper to taste.
Fill the mushroom caps with the mixture.
Cover and micro wave on HIGH for 3 minutes.
Mix the cornstarch (cornflour) to a smooth paste with the madeira and add the remaining sour cream (creme fraiche) and the mushroom cooking liquid.
Stir well and cook for 1 minute on HIGH.
Stir again.