Wash the plums, cut them in two, and remove the pits. Place them skin side down in the tart shell. Sprinkle the fruit with two tablespoons of the sugar.
Beat the rest of the sugar (6 tablespoons) with the egg. Stir in the flour and mix completely. Stir in the milk, butter, and Kirsch.
Pour the mixture over the plums and bake at 400°F for 45 minutes. Let cool completely. Serve cold.
Makes 6 - 8 servings.