Rinse the quinoa in a sieve. Put it into a saucepan with a pinch of salt and plenty of water. Bring to the boil, then reduce the heat and simmer for 15-20 minutes, until tender. Rinse with cold water to cool, then drain really well.
Beat the TREX and caster sugar together until light and creamy, then gradually beat in the eggs. Stir in the vanilla, quinoa, ground almonds, coconut and baking powder, then add the lime zest and juice.
Transfer the mixture to the prepared cake tin. Bake for 1 hour 15 minutes until set and golden. Place on a wire rack to cool completely. When cool, remove the lining paper (the cake has a crumbly texture, so handle carefully).
To decorate, beat the TREX and icing sugar together until light and creamy, adding 2tsp lime juice. Put into a piping bag and pipe whirls on top of the cake (or just use a teaspoon). Next, put the shreds of lime zest, remaining lime juice and caster sugar into a small saucepan. Cook for 2-3 minutes until syrupy. When cooled, spoon over the cake and serve, decorated with lime slices and shredded coconut.