The day before for the lavender tea, bring 750 ml of water and sugar to a boil in a saucepan. Stir until the sugar has dissolved. Add lavender and remove from heat.
Cover and let the tea cool down at room temperature.
After cooling, pour through a sieve. Pour the tea into a glass mug, fill up with the remaining water and lemon juice.
Stir well, season to taste and add sugar if desired.
Chill well in the refrigerator.
Just before serving, stir the lemonade again and add ice cubes to the jug. To serve, pour into chilled glasses and garnish each glass with a lavender flower.