Step 1Bring the zest, orange juice and sugar to boil in a large saucepan for 2 minutes. Add the plums. Simmer, stirring often, until the plums begin to soften but are not falling apart, 5 to 7 minutes. Transfer to a bowl. Stir in the rum, if using, and half the raspberries. (This can be made two days ahead and refrigerated. Serve warm. Gently reheat until just warm in a microwave oven or on a stovetop.)
Step 2Stir in the remaining raspberries.