Combine lavender, 1 cup water, and sugar in a saucepan. Bring to a gentle simmer, stir, then remove from heat. Let steep for 20 minutes.
Strain the syrup through a fine mesh strainer to remove lavender bits. Cool completely.
Juice the oranges and lemon. Strain if pulpy.
In a shaker or pitcher, add 1/4 cup lavender syrup, fresh orange juice, orange blossom water, and lemon juice.
Fill with ice. Shake or stir until well chilled.
Add sparkling water. Stir gently to combine.
Taste and adjust: add more syrup for sweetness or more lemon juice for tartness.
Pour into glasses over fresh ice.
Garnish with orange zest, lavender sprigs, or edible flowers.