Preheat the oven to 170C (fan) and grease and line a 9-inch cake tin.
Grind the walnuts in a food processor until they resemble fine crumbs. Be careful not to keep the motor running for too long or it will start becoming oily. Tip the nuts into a mixing bowl and whisk in the polenta, baking powder and a nice pinch of salt.
In a separate bowl, use an electric whisk to whisk the olive oil with the sugar for a minute or so. The sugar won’t dissolve but the mixture will become thicker and sludgy.
Add the eggs, one by one, whisking as you go and then the lemon/orange zest and vanilla until you have a nice velvety mixture and everything is well combined.
Fold the dry ingredients into the wet ingredients gently until everything is combined. If you are adding chocolate chips, fruit or anything else fold these into the batter now too or arrange the fruit on top (though it tends to sink whilst in the oven).
Pour the batter into the prepared cake tin and put in the oven for 20-25 minutes until golden on top and a cake tester comes out clean. Allow the cake to cool in the tin before digging in. Serve with a dollop of sweetened mascarpone or whipped cream.