Measure the cornflour into a bowl.
Blend in a little yoghurt, stir in the eggs and whisk with a hand whisk until smooth.
Mix in the remaining yoghurt and the mustard.
If using with a moussaka (or another oven-baked dish) the yoghurt sauce is now ready to be spread on top of the aubergines, sprinkled with cheese and cooked in a preheated oven for about 45 minutes. This will thicken the yoghurt and cheese sauce.
For a hot pouring sauce (for fish), whisk four tablespoons of yoghurt with one tablespoon of the cornflour and the mustard until smooth.
Blend in the remaining yoghurt.
Bring to the boil and stir constantly, until thickened and smooth.
While stirring, add the cheese, two egg yolks (not the whites), and season with salt and pepper.
At this point a squeeze of lemon juice can be added if desired.
Season with salt and pepper and pour directly onto the already cooked fish.