Cut plums in half and remove stones. Place the plum halves in a single layer on a large baking tray with tall sides (this is necessary so that the juice does not spill on to the bottom of your oven).
Sprinkle chopped walnuts o.n top
Preheat oven to 200 C / Gas 6; cook until plums are very soft, about 2 hours and 45 minutes. Transfer plums with all the juice into a large bowl; stir till smooth. Spoon into sterilised jars and seal with tight-fitting lids.