Rinse the blueberries and blackberries under cold water and drain thoroughly.

In a small saucepan over medium heat, combine the berries, granulated sugar, lemon juice, and lemon zest.

In a small ramekin, whisk the water and cornstarch together to form a smooth slurry.

Heat the berry mixture until the fruit begins to release its juices and reaches a gentle simmer, approximately 5 minutes.

Stir the cornstarch slurry into the saucepan and continue to simmer for 5-7 minutes, stirring occasionally to prevent scorching.

Once the liquid has thickened and the berries are tender but mostly intact, remove from heat.

Transfer to a glass bowl and allow to cool to room temperature; the mixture will thicken further as it cools.