Place water and fruit in a pan on the stove and bring to a boil. Simmer until very soft and juicy, 5-10 min. Sieve the fruit and measure the resulting juice. Return to pan (discard pulp) and add sugar, heating gently and stirring till dissolved. Measure out 2 teaspoons of cornstarch for each cup of juice, and blend with a little cold water. Add this mixture to the juice in the pan, and bring to the boil, STIRRING CONSTANTLY. I can't emphasize that enough or it will get lumpy and gross. Boil, stirring, until it thickens, about 3 min. Pour into glass bowl or serving glasses and chill until set. Decorate with nuts or whole fresh fruit, etc.
You can substitute peeled and diced rhubarb for about a third of the strawberries to give it a bit more zing.