Make the rice pudding: Put the rice in a medium saucepan, add the water, and bring to a simmer over medium-high heat. Add the milk and vanilla seeds, if using, and stir to mix well. As soon as the milk begins to simmer, reduce the heat to low, partially cover the pan, and cook, stirring occasionally, until the liquid is absorbed and the rice pudding is thick, about 45 minutes. Toward the end of cooking, stir the mixture more often to keep it from sticking or burning on the bottom of the pot. If using vanilla extract, add it at this point.
Transfer the rice pudding to a large bowl, cover, and set aside to cool, about 30 minutes.
Meanwhile, prepare the apples: Line a baking sheet with paper towels.
Peel and core the apple, cut it crosswise into ⅛-inch slices, then cut the slices into small squares, about ½ inch; they don't need to be perfectly uniform. Put the apples and ¼ cup water in a small saucepan, bring to a simmer over medium heat, and cook, partially covered, until the apples are tender, about 15 minutes. Using a slotted spoon, transfer the apples to the prepared baking sheet to drain and cool.
Make the cherry sauce: Put the cherries and their syrup in a small saucepan and heat over medium heat. Meanwhile, in a small bowl, mix the cornstarch with the water until smooth (your finger is the best tool for this; this mixture is called a slurry). When the cherry syrup begins to simmer, add the cornstarch slurry, lower the heat, and simmer the sauce, stirring constantly, until glossy and thick, 1 to 3 minutes.
Remove the pan from the heat and sprinkle the sugar over the surface of the sauce to precent a film from forming. Set the sauce aside to cool to room temperature.
Just before serving, put the cream and sugar in a large bowl and beat with a handheld electric mixer or a whisk until the cream doubles in volume and forms soft peaks, 3 to 4 minutes.
Fold the cooled apple slices into the rice pudding and then gently fold in the whipped cream until well combined, keeping as much volume of the whipped cream as possible.
Serve the rice pudding in individual bowls with about 2 tablespoons of the cherry sauce spooned over each portion.
Make ahead: The rice pudding, cooked apple slices and cherry sauce can be prepared one day ahead and refrigerated. Before serving, whip the cream, fold the components together, and serve at room temperature.
