Grease a 23 x 33cm Swiss roll tin
and line with nonstick baking paper.
Melt the butter in a saucepan,
stir in the flour and mustard, and cook over a low heat for 1
minute, then gradually add the milk, stirring until evenly blended.
Bring the sauce slowly to the
boil, stirring constantly until it thickens.
Cook over a low heat for 2
minutes.
Remove the pan from the heat,
leave to cool slightly, then beat in the cheese, egg yolks, herbs,
salt and pepper.
Whisk the egg whites until stiff
and fold into the sauce until evenly incorporated.
Pour the mixture into the prepared
tin and place in a preheated oven, 400F, gas mark 6, for 12-15
minutes until risen and firm to the touch.
Remove from the oven and set aside
to cool.
Reduce the temperature to 375F,
gas mark 5.
Meanwhile, prepare the filling.
Beat the cheese and half the oil
together until smooth and season with salt and pepper.
Heat the remaining oil in a frying
pan and fry the leek for 5 minutes.
Drain the spinach in a colander,
pressing with a wooden spoon to squeeze out the excess moisture, and
chop finely.
Add to the leeks, season with
nutmeg and cook gently for 5 minutes.
To assemble the roulade, turn it
out of the tin and carefully peel away the paper.
Spread the softened cheese and
then the spinach mixture on the roulade.
Roll up from a short end and place
on the oiled Swiss roll tin.