First, line a baking tray with baking paper and preheat your oven to 180C/160C fan/Gas 4.
Peel, core and chop your apples into slices and place them in a saucepan or frying pan. Remove the stones from all-but-4 of the plums and add them in too. Add your ginger, lemon juice and 100g of sugar and place on a medium heat.
Bring your fruit to a simmer, stirring slowly. It will begin to break down slightly (the plums first) – keep simmering until the fruit is at the stage you like. Taste a whole bit of fruit to check the sweetness, and add more sugar as required. Once ready, turn off the heat and set aside.
Unfurl (or roll out) your puff pastry sheet. Using a small bowl as a template, cut out circles about 10cm wide with a knife. You can re-use the offcuts, just pile them up and roll them out with a wee bit of flour, then use your template to cut out more circles. Place the discs on your lined tray.
Divide your stewed fruit mixture between each disc. Cut each of your reserved plums in half and remove the stone – place these flesh-side up on your tarts. Sprinkle a wee bit more sugar on top, if you like, to help them caramelise
Finally, dab a wee bit of milk around the puff pastry that’s at the edge of the fruit – this will rise up in the oven (hopefully!) and the milk will give it a golden brown shine.
Bake your tarts for about 20-25 minutes, or until the edges of the fruit and pastry are just starting to catch.
Plums have stones in. If you’re really lazy and can’t be bothered stoning them, you can just rip them apart and chuck them into your pan. Remember how many you added and then, as they fall apart, scoop the free stones out (they float).