Preheat oven to 325 degrees F. Beat eggs whites until frothy. While beating, add lemon juice and a few drops of cold water.
As the egg whites start to thicken, add half the sugar and half the sugar substitute.
In a separate bowl, beat the egg yolks with the other half of the sugar and sugar substitute. With the mixer running on low, slowly add the oil to the yolks while beating, until mixture turns light lemon-colored and thick.
Mix the potato starch, ground almonds, and baking powder together. Add to the egg yolks, and using a spatula, fold in by hand, alternating with the tea.
Carefully fold part of the whites into this mixture and then fold this mixture back into the whites.
Bake in a non-greased 10-inch round tube pan for about 30-40 minutes, or until an inserted toothpick comes out clean and the top springs back to the touch.