Preheat oven to 180C/160C fan-forced. Grease a 6cm-deep, 20cm-square cake pan. Line base and sides with baking paper.
Using an electric mixer, beat butter, sugar and lemon rind together until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Add flour, almond meal, mascarpone and lemon juice. Beat on low for 1 minute or until just combined. Spread mixture into pan. Level top.
Bake for 50 minutes to 1 hour or until a skewer inserted into centre of cake comes out clean. Stand cake in pan for 10 minutes. Turn, top side-up, onto a wire rack. Cool.
Make Candied lemons: Place sugar and 1 cup cold water in a large, deep heavy-based frying pan over medium heat. Cook, stirring, until sugar is dissolved. Bring to the boil. Reduce heat to low (see notes). Add half the lemon slices. Simmer, turning lemons occasionally, for 8 minutes or until pith of lemon is translucent. Using tongs, transfer lemons to a baking paper-lined baking tray. Repeat with remaining lemon slices.
Place cake on a serving plate. Arrange lemon slices on top of cake. Drizzle syrup from frying pan over cake. Stand for 10 minutes. Serve with double cream.