Preheat oven to 350 degrees F. Line a long loaf pan with baking paper.
Place boiling water and apricot jam in a glass dish. Cover with a plate or piece of foil. Let stand for 2 minutes. If the mixture does not blend easily, microwave for 30 seconds and mix with a wire whisk or fork and cool.
Sift the flours, cinnamon, baking powder and baking soda.
Mix the tea with the lemon zest and vanilla.
In a mixer on a high setting, beat the egg, slowly adding the oil and sugar substitute. Beat until the mixture thickens.
Add the apricot mixture, mixing by hand with a wooden spoon. Add the tea and flours, alternating between the two. Continue mixing until blended.
Spread in the pan and bake until the cake starts to turn brown and the sides begin to pull away, 20-30 minutes. Cool and garnish. Keep refrigerated or freeze.
To prepare the optional glaze, place sugar substitute in a cup or small bowl. Slowly add water, lemon juice and vanilla extract, a few drops at a time, mixing while you add, until the mixture is thick. Spoon or brush over the sliced bars. Sprinkle with walnuts if desired and freeze.