You will need 4 thick slices of panettone. Each should weigh about 60g.
Make a light lemon syrup by squeezing enough lemons to give 100ml of juice. This will probably be about 3, depending on their size and ripeness. Put the juice in a small saucepan with 4 tbsp of caster sugar and bring to the boil. Remove from the heat as soon as the sugar has melted and cool before use.