Preheat the oven to 180°C.
Roughly grind the walnuts in blender, they should remain lumpy.
In a large mixing bowl mix the flour with the grated orange peel, baking powder, salt and walnuts.
Add the orange juice, oil, egg and sugar. Mix gently but don't over-stir! You should get a wet batter, which is not too thick - if necessary, add more orange juice.
Grease and line the base of a cake tin. Pour the batter into it, let it spread out evenly then bake for 35-40 minutes until golden brown and an inserted skewer in middle comes out clean.
Take the tin out of the oven and let the cake rest for 10 minutes in the tin, then transfer it to a wire rack to cool completely.
Let rest for 10 minutes in the tin then transfer it to a wire rack to cool completely.
Serve with icing sugar dusted on the top.