Preheat an oven to 340.
Melt butter in sauté pan & soften leeks ( approx 10 min).
mix cream, egg, mustard, chives and two-thirds of the grated cheese.
Take the leeks off the heat and stir in the mixture, then leave to cool.
Grease and flour tart pan. Press in crust.
Evenly spread cooled leek mix into crust. Sprinkle remaining cheese on top.
bake for 25-30 minutes.
Best served room temperature