Place lemons in a saucepan; cover with water. Bring to the boil. Top lemons with a lid or small plate to keep submerged. Reduce heat to medium; simmer for 30 minutes or until tender. Drain. When cool enough to handle, tear open; discard pips.

Preheat oven to 160°C/325°F. Lightly grease a 23cm (9¼in) springform pan; double line base and side with baking paper.

Process lemons, ricotta, honey, oil, vanilla and egg yolks until smooth.

Combine polenta, almond meal and baking powder in a large bowl; stir in lemon mixture. Whisk egg whites and a pinch of salt with an electric mixer until soft peaks form; fold through polenta mixture. Spoon into pan; smooth top.

Bake cake for 50 minutes or until a skewer inserted into the centre comes out clean.

Make rosemary lemon syrup and whipped lemon ricotta.

Spoon half the hot syrup over warm cake in the pan; stand until cooled. Transfer cake to a plate; serve with remaining syrup and whipped lemon ricotta.

Place honey, ½ cup (125ml) water and rosemary in a small saucepan over high heat. Bring to the boil; cook for 4 minutes. Add lemon juice, vanilla and lemon slices; simmer for 2 minutes.

Process ricotta, honey, lemon juice and 2 tablespoons water until smooth. Refrigerate until required.