Tip the quinoa into a saucepan, add a pinch of salt and cover well with water - at least double the quinoa volume.
Put the saucepan on the heat and bring to the boil, then reduce to a simmer. Cook gently for approximately 20 minutes until the quinoa tastes tender and you can see that the grains have popped open, revealing little white kernels.
Rinse the quinoa under a running cold tap to cool quickly, then leave to stand in the sieve for 10 minutes to ensure it’s drained really well - soggy quinoa will make your cake too wet.
Preheat the oven to 180C (160C fan / 350F).
Put the oil, caster sugar, eggs and vanilla in a large bowl and whisk together.
Add the cooked quinoa, ground almonds, baking powder and the orange zest. Whisk lightly to combine.
Pour the cake batter into the prepared cake tin.
Bake for 1 hour until set and golden. A skewer inserted into the centre of the cake should come out clean.
Cool for 20 minutes in the tin, then release from the tin carefully and place on a wire rack to cool completely.
To serve, cut into slices with a sharp, serrated knife.