To make the pastry, mix together the flour,salt, butter or margarine and lard until mixture is the consistency of coarse meal.
Add sherry, a little at a time, until pastry forms a stiff ball.
Chill.
Roll out on a lightly floured board and line the bottom of a 9 inch (23 cm) pie pan.
Press ricotta cheese through a fine sieve.
Add flour, sugar and lemon juice and mix well.
Beat egg yolks until thick and light yellow colored.
Gently mix into the ricotta mixture.
Fold the whipped cream into the mixture.
Beat the egg whites until stiff and gently fold into the ricotta mixture.
Pour into an unbaked pastry shell.
Bake in a 300°F (150°C) oven for one hour.
Turn off the heat and leave the pie in the oven for another hour with the door closed.
Remove from oven and cool.