Place 2 lemons in a medium saucepan. Cover with water. Bring to the boil. Reduce heat to medium-low. Simmer for 1 hour 30 minutes or until very soft. Drain. Cool to room temperature.
Meanwhile, thinly slice remaining 2 lemons.
Preheat oven to 180C/160C fan-forced. Grease and line a 6cm-deep, 20cm round cake pan.
Place butter and 2 tbs honey in a small saucepan over medium heat. Cook, stirring, until melted and smooth. Bring to a simmer. Pour over base of prepared pan. Arrange lemon slices in base of pan, slightly overlapping.
Coarsely chop cooked lemon, removing and discarding seeds. Place chopped lemon in a food processor. Process until pulpy. Add eggs, remaining ⅓ cup honey, vanilla and oil. Process until well combined. Add baking powder, almond meal and coconut. Process until just combined.
Pour into prepared pan. Bake for 45 to 50 minutes or until a skewer inserted into the centre comes out clean. Cool cake in pan for 15 minutes. Turn out onto a wire rack to cool.
Drizzle cake with extra honey. Sprinkle with pistachios. Serve with mascarpone.
