Preheat your oven to 180C/350F/Gas mark 4 and either lightly grease or line a 2-pound loaf tin and place it to one side.
In a medium-sized mixing bowl combine the flours, baking powder, cinnamon, salt and sweetener if you are using it. Mix everything together, then place to one side.
In a medium sized jug/bowl add in the mashed banana, vanilla extract, egg, melted spread and the 3 tablespoons of milk and gently whisk until everything is well combined.
Pour the liquid ingredients into the dry and with a gentle hand fold everything together.
Fold in 80% of the rhubarb slices and pour the batter into your loaf tin. Sprinkle the remaining rhubarb slices over the top and place in your warm oven.
Check on your loaf after 40 minutes if a skewer does not come out clean then give it a few more minutes. I have a slow oven and I found 45 minutes hit the sweet spot.
Take the loaf out of the oven and leave it in the tin for 10 minutes before transferring it to a wire rack to fully cool down.
Kept in an airtight tin this loaf is good for 5 days or it can be frozen for up to 2 months.