Mix sugar and water in a saucepan and bring to a boil, ensuring that all sugar is fully dispersed in the water.
Add the dried Hibiscus flowers and take off from the heat. Let the flowers infuse for about 15 minutes.
Optional, if using Agar agar: add the agar to the syrup once the flowers have infused and been sieved off. Bring the syrup to the boil again for a couple of minutes, let cool down a little and mix with the purée as per below as quickly as possible.
Turn the raspberries into fruit purée (using a handheld mixer or similar) and add the lemon juice.
When the infusion is finished, sieve off the Hibiscus flowers and add the Hibiscus sugar syrup to the raspberry purée. Mix well - your base is ready.
Churn in your ice cream machine until ready, or still-freeze using your freezer if you have to.