Clean the fruit well; chop rhubarb, if using, into 1-inch pieces. Place into a nonreactive saucepan and cover with 3 cups water. Simmer over medium low heat until the fruit falls apart. Remove from heat and strain juice through cheesecloth or a fine-meshed sieve; dispose of the pulp and seeds.
Return juice to heat, stir in sugar, and bring to a low boil; reduce heat to medium and add the dissolved corn starch liquid to thicken. Stir the mixture constantly until it begins to gel to the consistency of a heavy syrup. Remove from heat, pour into individual serving bowls, cover with plastic wrap, and refrigerate for at least two hours