Split the leeks lengthwise and peel off the largest outer leaves, making a total of 12 in all. Chop the remaining leeks. Blanch the outer leaves in boiling water for 2 minutes until soft, then remove.
Use the outer leaves to line a greased 1lb loaf tin. Take one leaf, cover a section of the base of the tin, then the side, leaving the remainder of the leaf to fold over later. Repeat with the next leaf, slightly overlapping the one before. Repeat using all the leaves until the tin is completely lined.
In a separate pan, melt the butter and saute the chopped leeks for 2-3 minutes, then add a little water and simmer for 10-15 minutes or until soft. Puree using a food processor or blender. Add the fromage frais and beaten egg and mix well. Season well.
Pour the mixture into the tin and fold over the leek leaves so that the mousse is enclosed.
5. Set the tin in a baking dish of hot water and bake at 325 F/170 C/gas mark 3 for 40-45 minutes, or until the mixture feels firm to the touch, Remove from the oven and leave to cool in the tin. When absolutely cold, slice with a serrated knife. Garnish with fresh chives. Serve with Fresh Tomato and Pepper Relish