While the French Toast is cooking, make the blueberry topping. In a saucepan, combine sugar, cornstarch, vanilla, lemon zest, lemon juice, and water. Stir and place over high heat. Bring to a boil, then lower the heat and simmer for a minute. Set aside to cool slightly.
Top the french toast with the blueberry topping. Optionally add some freshly grated lemon zest on top and serve immediately.