For the cake, preheat the oven to 170C/150C Fan/Gas 3.
Line the base of a deep 20cm/8in round tin with baking parchment and grease with butter.
Combine the almonds, polenta, flour and bicarbonate of soda in a bowl and set aside.
In a free-standing mixer, beat the butter until smooth, then add the honey and grapefruit zest.
Gradually beat in the eggs and reserved dry ingredients. Pour the batter into the tin and bake for approximately 30 minutes.
For the citrus syrup, mix the grapefruit and orange juice and honey in a saucepan and heat gently until the honey has melted. Bring to the boil and simmer until temp reaches 105C on a sugar thermometer. Remove from the heat.
Once the cake is baked, set it aside to cool. Prick the surface of the cake with a skewer and drizzle with half the citrus syrup.
For the candied peel, boil a kettle full of water. Use a potato peeler to remove slices of the citrus fruit skin.
Add the peel to a saucepan and cover with boiling water. Boil for 2 minutes, then remove the peel and discard the water. Repeat this process twice until the bitterness has been removed from the peel.
Add the remaining citrus syrup to a saucepan. Add the peel and cook for 5-10 minutes until translucent. Set aside to cool.
For the maple cream, heat the maple syrup in a small pan and boil until reduced by half. Set aside to cool. Beat the mascarpone until smooth, then add the reduced maple syrup.
To assemble the cake, cover with the maple cream and arrange the candied peel on top.