Preheat the oven to 180°C.
Remove the stones of the plums then cut the flesh into wedges. Roughly chop the prunes.
Lay them into a small shallow baking dish and drizzle with half of the maple syrup or honey. Sprinkle half of the cinnamon over it.
Put the walnuts into a small food processor and whiz until you get a coarse powder. Transfer this into a mixing bowl. Add the oats and remaining cinnamon to the food processor, then crumble the butter/spread over it and process until you get a breadcrumb-like mixture. Add this to the walnuts, then drizzle with the remaining maple syrup and combine.
Sprinkle the crumbs over the fruits, pushing slightly down with a spoon. Drizzle the top with more maple syrup or honey if you like, then bake for about 20 minutes, until the top is golden.
Serve with Greek yoghurt mixed with maple syrup/honey if dairy is not an issue for you (or custard, vanilla ice-cream for a less healthy version).