Make a slurry by combining cornstarch with 2 Tablespoons water.
In a medium sauce pan over medium-high heat, combine the blueberries, remaining water, sugar, lemon juice and lemon zest. Bring to a gentle simmer and allow berries to break down, about 8 minutes.
Whisk in the slurry and continue to gently simmer the mixture until the sauce thickens, about 2 minutes. Transfer the sauce to a container and let it cool completely before serving or storing it, covered, in the fridge for up to 3 days.
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