1) prepare (or buy) one open pie crust.
2) cut off and discard tough, dark green leaves of leeks.
3) using only white stem and a few inches of tender green of the leeks, cut lengthwise in half, and again, and swish in a bowl of cold water (to remove dirt/sand).
4) chop cleaned leeks very finely, or shred in food processor
5) saute leeks in 1 T. butter, and set aside in large bowl.
6) prepare White Sauce, gradually adding 3 T. butter, 3 T. flour, and 1 cup cream while whisking together over medium heat, until thickens.
7) pour white sauce into bowl with sauteed leeks; add shredded parmesian cheese, nutmeg, salt and pepper; and stir together until well mixed.
8) pour contents of bowl into pie crust.
9) bake at 375 for 30-40 minutes, until golden brown and slightly firm to the touch.