Put blueberries, sugar, water, and lavender in a small saucepan.
Heat over medium, stirring now and then, until it comes to a gentle simmer.
Once the berries are bursting and the sugar’s dissolved (about 5 minutes), take it off the heat.
Let the syrup cool down for 15-20 minutes. Then strain it through the mesh strainer.
Toss the solids. Pour the clear syrup into a bowl or bottle.
In your shaker, combine 1 ounce blueberry lavender syrup, 1 ounce lemon juice, and some ice. Give it a gentle shake.
Pour that into a champagne flute or your glass of choice.
Top with 3-4 ounces chilled prosecco and stir gently with the bar spoon.
Add extra blueberries and a sprig of lavender or mint if you’re feeling fancy.