Photograph: Kate Whitaker
Preheat the oven to 180°C, fan 160°C, gas 4. Lightly butter an 18cm square tin, 6-7cm deep, and line with nonstick baking paper. Beat together the butter, sugar, eggs, flour, baking powder, polenta and lemon zest and juice in an electric mixer until combined. Spoon the mixture into the tin and bake in the preheated oven for 45 minutes, or until risen, firm to the touch and golden.
Meanwhile, to make the lemon syrup, sift the icing sugar into a bowl and whisk in the lemon zest and juice; set aside.
When the cake is ready, remove from the oven and make holes all over the surface with a fork. Spoon the lemon syrup over the cake, allowing it to seep in between spoonfuls – it will seem like a lot, but gluten-free flour is very absorbent.
Top with lemon slices that you have simmered in the caster sugar and water for 45 minutes until tender.