1. Quarter each plum and remove the stone. If using large plums you might want to half each segment again.
2. Put the diced apples and plums in a saucepan along with the water, cinnamon, lemon zest and 50g of the caster sugar. Cover and simmer on a low-medium heat until the apples have softened and the sugar has melted.
3. Heat your oven to 200 C/Gas Mark 6.
4. In a large bowl, put the plain flour, butter, soft brown sugar and remaining caster sugar. Combine and rub together with your fingertips until your have a fine crumble consistency.
5. Put the apple and plum filling in the bottom of a baking dish and cover with the crumble topping. Bake in the oven for 25-30 minute or until the top goes golden brown.
Spruce it Up
When you make your crumble topping, sprinkle in a handful of flaked almonds for even more crunch.