In a medium saucepan, combine the blueberries, sugar, and lemon juice. Heat over medium heat while stirring until the sugar dissolves.
Mix the cornstarch with 2 tablespoons of water to create a slurry, then add it to the saucepan. Cook and stir until the mixture thickens, about 2-3 minutes.
Remove from heat and stir in the lemon zest. Let cool before serving.
Notes
This topping pairs perfectly with cakes, pancakes, and yogurt. Store in an airtight container in the fridge for up to one week.