In a saucepan, bring 100 ml of water with the lemon juice, sugar and agar-agar to the boil and simmer for 3 minutes. Remove from heat and stir in yogurt and cream.
Pour the mixture into 4 cold rinsed molds and leave to set in the refrigerator for about two hours.
A fruit puree goes perfectly with it - quickly prepared from frozen berries or fresh fruit.