Begin with making the raspberry gel. Place the raspberries and lemon juice in a saucepan. Use a stick blender to blitz until smooth. Stir in the sugar and agar powder. Warm over medium heat for 2 minutes or until starts to come to a simmer and the sugar dissolves (do not boil). Pour onto a baking tray. Cool slightly, then place in the fridge for 1 hour to set.