Preheat oven to 325°F (163°C).
Cream the butter and sugar well and add the egg yolks, one by one. Beat in flour and buttermilk and add the lemon juice and nutmeg with a pinch of salt. Whip the egg whites until stiff and fold into the filling. Pour the filling into the partially baked pie pastry and cook in the middle level of the preheated oven until the custard is set and slightly brown, about 1 hour. Serve topped with fresh fruit such as peaches, strawberries, or blueberries and slightly sweetened whipped cream. Fresh fruit sauces are also excellent with this (see recipes).