Cook Blueberries: Into a pot add a bit of water, the blueberries (no need to defrost if frozen), the sugar, a pinch of salt, and the lemon zest if using; stir and set over medium heat. Bring to a simmer, stirring occasionally, and watch closely at first until the berries release their juices.
3 cups blueberries, ¼ cup white sugar, lemon zest, pinch salt
Adjust Flavor: Remove the pot from the heat and taste; add more sugar if it needs sweetness, a squeeze of lemon juice for brightness (especially if you skipped the zest), or a pinch more salt until the flavor feels balanced to you.
2 teaspoons lemon juice
Thicken With Slurry: In a small bowl combine the water and cornstarch and stir until smooth, then pour half of this slurry into the pot while stirring and return the pot to low heat. Continue stirring until the mixture thickens; if it’s still too thin (not spoonable consistency), add another half to three quarters of the remaining slurry (or make more if needed) and repeat. Once the consistency is good, cook for about 1 minute on low to remove any raw cornstarch taste.
2 tablespoons cornstarch, 2 tablespoons water
Finish And Adjust: Take the compote off the heat one last time and taste, adding a touch more sugar or lemon juice if needed to reach the flavor you like.
Cool And Store: Use the compote immediately while warm or let it cool and refrigerate for later use.