Butter the ramekins generously. Place the ramekins in a flat, deep roasting dish and set aside.
Heat a drizzle of olive oil in the large frying pan. Cook the mushrooms until golden brown. Season lightly and set aside to cool.
Once cool, divide the cooked mushrooms amongst the buttered ramekins.
Melt butter in a medium sized saucepan and scatter over the flour. Cook the mixture, moving it around the pan with a spatula.
Switch to a whisk and pour in the milk very gradually while constantly whisking to keep the mixture smooth. Cook over medium high heat until béchamel thickens.
Remove from the heat and add the parmesan, Dijon and nutmeg. Season with salt and pepper, to taste.
Beat in the egg yolks, one at a time, until fully incorporated. Set aside.
Beat the egg whites until soft peaks form. Stir one third of the egg whites into the cheese mixture thoroughly.
Very gently fold the remaining two thirds of the egg whites in, keeping as much air in the mixture as possible.
Divide the mixture evenly between the buttered ramekins. Pour enough boiling water into the tin to come halfway up the sides of the ramekins.
Bake the soufflés for 15 minutes until puffed up and lightly golden.
Remove from the oven, and allow to cool.