Preheat the oven to 140C/280F degrees and line a 1lb loaf tin with parchment paper.
In a medium bowl, beat the butter until smooth, then add in the monkfruit.
Beat in the eggs, one at time and add the egg white.
Add the Greek yogurt, lemon juice, lemon rind, vanilla extract and almond milk and mix well.
In another small bowl, add the oat fiber, lupin flour, whey protein powder, salt, baking powder and baking soda. Lightly whisk until mixed.
Add the dry ingredients to the bowl of wet ingredients and stir until combined.
Spoon the cake mixture into the prepared loaf tin and bake for 45 to 50 minutes.
Allow to cool in the tin for 10 minutes before removing from the tin.