You will need
Preheat oven to 180C/350F/Gas 4.
Sift together polenta, flour, baking powder and salt.
Put yogurt, oil, lemon rind and 30ml/2tbsp lemon juice in a jug and stir to combine.
In a separate bowl, beat eggs, egg whites and half the sugar until creamy. Beat in yogurt mixture until smooth then stir in dry ingredients until just combined. Do not overmix.
Pour batter into a lightly oiled 1.2L/2pt loaf tin lined with baking paper. Bake for 40-45 minutes or until a toothpick inserted in the centre comes out clean.
Meanwhile, put remaining sugar in a saucepan with 200ml/7fl oz water and rosemary. Simmer for 10 minutes. Cool and strain.
When cake is cooked, put it on a cooling rack for 15 minutes then invert and remove baking paper. Prick all over with a toothpick and drizzle half the rosemary syrup over. Cool completely.
To serve, slice cake and arrange on plates. Scatter raspberries around and drizzle more syrup over them. Decorate with rosemary sprigs.