Put the saskatoons in a medium sized saucepan.
Combine ¼ cup (60ml) of the water with the cornstarch in a small bowl and set aside. Add the remaining 2 cups (480ml) of water to the saskatoons in the saucepan.
Bring the saskatoons to a boil, then reduce the heat to medium and cook them, covered, for 10 minutes.
Scoop out 1 cup (250ml) of the cooked berries with a slotted spoon and set them aside.
Put the remaining berries and juice through a food mill, using the fine disk, and set over a large bowl or the empty saucepan. (Don't clean the saucepan yet, you'll need it again.) Make sure to scrape all the pulp off the bottom of the food mill and add it to the juice.[*If you don't have a food mill, you can push the berries through a coarse sieve (not a colander) with the back of a spoon. It takes a little more time and effort, but it will work just as well. See the notes, below.]
Measure the juice/pulp mixture. You will need 3½ cups (840ml) in total. If you have less than that, add water to make up the amount. Return the juice/pulp mixture to the saucepan.
Add the reserved whole berries, sugar, lemon zest, and lemon juice to the saucepan.
Bring the soup to a boil. Stir up the cornstarch slurry, which will have settled, and pour it into the boiling soup, stirring constantly with the other hand as you pour so the cornstarch doesn't clump up.
Boil for 30 seconds, then remove the saucepan from the heat and stir in the rosewater.
Allow the soup to cool, then chill it until cold - at least 3 to 4 hours.
Serve the saskatoon soup as an appetizer with a dollop of plain yogurt in the center, or serve it for dessert with a scoop of vanilla ice cream in it.
Serves 4 as an appetizer or 6 as a dessert.