In a medium-sized saucepan over medium heat, place berries, 2 tablespoons sugar, lemon juice, and zest. Cook for about 5 minutes, mixing occasionally until the liquid (which is the juice released from the fruit) thickens. At any point, you can add water if the sauce it too thick, or more sugar to make it sweeter.

In a small cup, whisk together cornstarch and water until smooth, then add it to the saucepan. Let the mixture simmer for a few minutes longer until it reaches the desired consistency. Sauce will thicken further as it cools.