- Combine sugar, water and dried lavender flowers into a pot.
- Place the pot on the stove and bring the mixture up to simmer.
- Remove from heat and let rest for 20 minutes, then strain out lavender.
- Pour freshly-squeezed grapefruit juice, freshly squeezed lemon juice and lavender syrup into shaker and shake with ice.
- Strain into a glass with fresh ice.
- Top with soda and garnish with a half-wheel of grapefruit.