In a medium-sized pot over medium-high heat, combine frozen blueberries, lemon juice, water, and sugar. Stir until sugar is dissolved and mixture comes to a simmer.

Muddle berries, mashing them in a pot. Add the sage. Simmer for 1–2 minutes.

Remove from stove and let sit overnight to infuse the sage.

Strain the blueberry mixture, pressing down on the berries, allowing them to release a colorful juice.

Place blueberry–sage syrup in a large punch bowl or glass container with a spigot.

Add gin and prosecco and garnish with a few sage leaves and lemon slices.