Lemon Pie

From 1911:

Take the juice and grated rind of one lemon, one cup of boiling water, one cup of sugar, one tablespoon of flour, yolk of one egg; dissolve the flour in a little water and boil all together.
Line a plate with crust and bake.
When done, fill with the above mixture; beat the whites to a stiff froth, add sugar to taste and spread over the pie.
Set in the oven until lightly browned.