Method

To make the white sauce :

•  In a medium saucepan, heat the butter over medium-low heat until melted. Add the flour and stir until smooth. Cook until the mixture turns a light, golden sandy colour, about 6 to 7 minutes.
• Meanwhile, heat the milk in a separate pan until it almost boiling. Add the hot milk to the butter mixture ½ cup at a time, whisking continuously until very smooth.
• Cook 10 minutes over a low heat, stirring constantly, then remove from heat. 
• Season with salt, mustard and nutmeg, set aside until ready to use.

Bake leeks :

• Preheat the oven to 200C.
• Half fill a suitable size pot with water and season with salt. Bring to the boil whilst you prepare the leeks.
• Trim off most of the dark green of the leek and tidy the other end if need be, but remember to keep the root end together.
• Wash the leeks well under running cold water and drain.
• Cook in the boiling water until tender (5-8 minutes) remove carefully as you want to keep the leeks whole.
• Drain well on a clean cloth, gently squeezing out excess moisture.
• Lay the leeks either in one large oven proof dish or in individual ones.
• Coat with the white sauce and sprinkle generously with the cheese and a little cracked pepper.