Preheat the oven to 200 C, 180 Fan, Gas 6. Halve and stone the plums, then arrange in a single layer, cut side up, in an ovenproof dish.
Mix together the sugar and cinnamon and sprinkle over the plums. Dot with the butter then scatter the lemon zest on top.
Put in the oven and bake for 25-30 minutes, basting with the juices a couple of times. When the plums are done they should be very tender but still hold their shape.