Preheat the oven to 220°Heat the oil in a pan and fry the mushrooms for 4 – 5 minutes.
Add the spring onions and cook for another minute. Set aside and reserve one third for garnish.
Melt the butter in a pan and stir in the flour and cook for 3 minutes, mixing well.
Add the hot milk and stirring continuously, keep stirring, for 2-3 minutes.
Remove from the heat and stir in the mustard and cheese.
Allow to cool slightly and stir in the egg yolks and reserved mushroom mixture. Season well with salt and pepper.
In a spotlessly clean bowl, whisk the egg whites, using an electric whisk, until soft peaks form.
Carefully fold the egg whites into the mushroom mixture.Arrange the toast slices onto a baking sheet and spoon the mixture onto the toasts.
Bake for 5 – 6 minutes or until golden and puffy. Serve at once topped with the reserved mushrooms and some parsley.