Put the whole orange in a small saucepan with cold water. Bring to a boil and simmer for an hour. Remove the orange and let it cool.
Preheat the oven to 340ºF (170ºC) and prepare a loaf pan by buttering and lining it.
Break the cooled orange into pieces and blitz in a food processor until you get a rough pulp.
Add the eggs to the orange puree and blend gently to combine.
Incorporate the brown sugar, almond meal, lemon juice, and baking powder. Blend until well combined.
Pour the batter into the prepared loaf pan, cover with foil, and bake for 40 minutes.
Remove the foil and bake for an additional 10 minutes or until a skewer comes out clean.
Allow the cake to cool completely in the pan before inverting onto a plate and decorating.