In a medium saucepan, combine water, granulated sweetener, and freshly squeezed lemon juice. Stir over medium heat until the sweetener dissolves, about 2–3 minutes.

Add fresh or frozen blueberries to the saucepan and reduce heat to low-medium. Simmer for about 15 minutes, stirring frequently.

Gently mash the blueberries with a fork or potato masher. Increase heat to medium and simmer for an additional 8–10 minutes until slightly thickened.

Remove from heat and strain the mixture through a fine mesh strainer into a bowl or container. Let it cool before refrigerating.

Chill the syrup in the refrigerator for 1–2 hours.

Fill a tall glass halfway with ice cubes, pour in club soda until three-quarters full.

Drizzle in the chilled blueberry syrup over the club soda.

Add a splash of half & half and swirl gently to mix. Top with whipped cream if desired.