Caramelized lemon slices are simply slices cooked in sugar caramel. To do this, you let the lemon slices cook in the sugar syrup until the syrup starts to caramelize. Once you get the caramel to a light amber hue, turn off the heat and remove the lemon slices onto a parchment-lined tray. Once the sugar syrup starts to caramelize, it will darken very quickly, hence keep a close tab on the sugar syrup and the lemons after 30 minutes of cooking.