Nonni's Lemon Pie Filling

This recipe from my grandmother is delicious, super easy and made from real lemons.  A whole recipe will fill 24 tarts or a 9" single pie crust.  You can purchase a frozen pie shell, bake it, let it cool and then fill it.

Filling:
3 Tbs flour
3 Tbs cornstarch
1/2 cup white sugar
pinch of salt
1/2 cup cold water
2 cups of boiling water
1 TBS of butter (unsalted)
3 egg yolks (mix with a fork)
Juice of 1 lemon
grated rind from 1 lemon

Directions:
Add about 2 inches of water to a double-boiler. Or use a pot, and place a stainless steel bowl over it.
Do not let the bowl touch the water. Boil the water. In the top portion, add the flour, sugar and salt. Gradually stir in the cold water, then add the boiling water.

Turn the heat down to a simmer and mix until thickened.  (about 5-8 minutes).

Add the mixed yolks and continue stirring till thick.

Add the lemon juice and rind and remove from the heat.

Add the butter.  Taste, and you can add more lemon juice if you like it more tart.  Cool.

Fill the cooled tart shells, or pie shell with the cooled lemon filling.  Serve topped with whipped cream.