Line a 12-section bun tray with paper cake cases. Cut one lemon into 12 thin slices. Put with the whole lemon in a small saucepan and cover with boiling water. Simmer very gently for 20–30 minutes until the slices are tender. Drain the slices and reserve. Cook the whole lemon for a further 15 minutes until soft and squashy. Drain and leave to cool.
Halve the whole lemon and discard the pips. Roughly chop, put in a food processor and process to a purée.
Mix the ground hazelnuts in a bowl with the flour, baking powder and poppy seeds. Mix the eggs with the lemon purée, agave nectar or sugar and melted butter and add to the dry ingredients with the sultanas. Stir until evenly combined.
Divide the cake mixture between the paper cases and place a reserved lemon slice on top. Drizzle each lemon slice with a little extra agave nectar.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and lightly browned. Transfer to a wire rack to cool.