Method

1. Preheat the oven to 220C/425F/Gas 7.

2. Gently melt the butter in a pan and stir in the flour to make a roux.

3. Gradually whisk in the milk then remove from the heat and stir in the egg yolks.

4. Stir in the cheese and mustard to combine.

5. In a separate clean bowl, whisk the egg whites to form stiff peaks.

6. Fold the egg whites into the cheese mixture and pour into a greased ovenproof dish.

7. Bake in the oven for 10-12 minutes, or until well risen and golden. Serve at once.