Preheat the oven to 190 C, 170 Fan, Gas 5. Put the damsons in a pie or gratin dish and mix with the apples. Drizzle over the honey and sprinkle with caster sugar.
Make the topping. Mix the flour, ground nuts and sugar then rub in the butter until the mixture resembles fine breadcrumbs. Sprinkle thickly over the fruit.
Scatter over a little extra demerara sugar. Do not press down. Bake for 25-30 minutes until lightly browned, with juice bubbling up around the sides. Serve immediately.