Rinse the quinoa in a sieve. Put it into a saucepan with a pinch of salt and double the volume of water. Bring to the boil, then reduce the heat and simmer for 10-12 minutes. Rinse with cold water to cool, then drain really well.
Preheat the oven to 180°C/Fan 160°C/Gas Mark 4. Grease and line a 20cm round cake tin.
Beat the dairy-free spread and Tate & Lyle Golden Caster Sugar together until light and creamy, then gradually beat in the eggs. Fold in the vanilla, quinoa, ground almonds and baking powder. Stir in the lemon zest and juice.
Transfer the mixture to the cake tin and level the top. Bake for 40-45 minutes until set and golden. Cool for 20 minutes in the tin, then turn out carefully and cool completely. Remove the lining paper.
To make the topping, put the lemon zest and juice into a small saucepan with the lemon slices and Tate & Lyle Icing Sugar. Add 3 tbsp water and heat gently until dissolved, then simmer for 2 minutes. Spoon over the cake and cool for a few minutes. Serve, sprinkled with the almonds and a little icing sugar.