How to Make It
In a large saucepan, cover the apples and the plums with the water and bring to a simmer. Cover and cook over low heat until the apples and plums break down, about 5 minutes. Drain the fruit in a fine sieve set over a bowl; reserve 2 tablespoons of the cooking liquid in a small bowl. Using a wooden spoon, press the fruit through the sieve into the bowl to make a fine puree. Discard the skins.
In a large saucepan, combine the apple-plum puree with the sugar, lemon juice, cinnamon and nutmeg. Simmer the puree over moderately low heat, stirring frequently, until the sugar dissolves, about 3 minutes. Stir in the reserved 2 tablespoons of cooking liquid. Scrape the applesauce into a bowl, cover and refrigerate for 1 hour, until chilled.