Preheat the oven to 180°C, gas mark 4. Lightly oil a 20cm x 14cm ovenproof dish.
Arrange the plums cut-side up in the dish and scatter over half the dates. Cover with foil and bake for 15 minutes.
Meanwhile, put the apple, remaining dates and 150ml water in a saucepan set over a medium heat. Bring to a simmer and cook for 6-8 minutes, until the apple is tender. Tip into a blender and blitz to a smooth purée, loosening with a splash of water if needed.
Combine the oats, pumpkin seeds and almonds in a bowl, then add the apple purée and oil. Stir well until the mixture comes together.
Remove the plums from the oven (discarding the foil) and top with the oat mixture. Bake for 15 minutes, until the topping is golden.