Preheat the oven to 200 degrees C and grease and flour a tart tin.
In a saucepan combine the water and honey and mix over low heat until honey has dissolved. Bring to a simmer and add the plums. Simmer about 5-10 minutes until plums are slightly softened but still firm. Drain and let cool, discarding the honey mixture.
In a saucepan combine the diced apples and 1 cup water. Simmer 10 minutes or until apples are soft. Blend to a rough puree, or use a stick blender.
Roll out the pastry until it is just larger than the tart tin. Place inside the tin, trim the edges and prick the bottom all over with a fork.
Pour in enough of the blended apples to cover the base. Top with the plum quarters in a circular pattern, skins downwards. Sprinkle with the brown sugar.