Put the shallot in a casserole with 1 tablespoon of butter, cover and microwave on HIGH for 2 minutes.
Wipe the mushrooms with a damp cloth or kitchen paper and cut into thin slices.
Pour the lemon juice over.
Add the mushrooms and 3 (2) tablespoons of water to the casserole, stir well and add salt and pepper to taste.
Cover and microwave on HIGH for 5 minutes.
When the mushrooms are cooked, drain and set aside the Put the remaining butter into a bowl and melt on HIGH for 1 minute.
Stir in the flour until smooth.
Add the reserved liquid from the mushrooms.
Microwave on HIGH for 1 minute, stir and microwave for a further 1 minute on HIGH.
Add the sour cream (creme fraiche) and microwave a further 1 minute on HIGH.
Taste and adjust seasoning.
Line the base of the microwave oven with kitchen paper.
Put the 4 vol au vent cases on it and heat them on HIGH for 11/2 minutes.
Cut the ham into cubes and add to the sauce.
Mix the egg yolk with the cognac and pour over the mushrooms.
Stir in the sauce.
Fill the vol au vent cases with the mixture