Add 1 cup water and ½ cup sugar to a small saucepan.
Stir in 2 teaspoons dried lavender buds and ½ teaspoon ground turmeric.
Heat mixture over medium until sugar dissolves.
Let it simmer for about 3 minutes.
Remove from heat and let cool for 15 minutes.
Strain syrup into a bowl with a fine mesh strainer. Discard solids.
Add ice to a shaker or mason jar, then pour in 2 tablespoons syrup and 2 tablespoons lemon juice.
Cover and shake well for about 15 seconds.
Strain into a highball glass with ice.
Top with ½ cup tonic water and stir gently with a bar spoon.
Garnish with a lemon wheel or lavender sprig if you like.