Preheat the oven to 180°C fan. Halve and stone the plums, then place cut side up in a roasting tin or ovenproof dish in which they can lie snugly in a single layer.
Crack the cardamom pods, then tip the seeds into a pestle and mortar and grind them to a fine powder. Mix with the sumac, honey, orange zest, lemon juice and water, then drizzle this mixture over the fruit. Bake for about 20 minutes, or until the plums are almost collapsing.
Meanwhile, make the candied pistachios. Line a baking tray with baking parchment, and have it ready next to the stove. Put the sugar, cinnamon, water and 1⁄8 teaspoon of salt into a large sauté pan, on a medium heat. Cook until the sugar has almost dissolved, stirring occasionally. Add the pistachios and continue to cook for 3–4 minutes, stirring occasionally (to avoid the nuts browning too much), until all the liquid has evaporated and you can see crystallization forming on the nuts. Tip the nuts on to the lined tray, and quickly spread them out to separate them. Leave to cool completely.
Place the cream, honey and ground cardamom in the bowl of a free-standing mixer, with a whisk attachment in place, and whip the cream on a medium speed for about 3 minutes, or until soft peaks form.
When ready to serve, divide the fruit and whipped cream between four plates and add a generous scattering of the candied pistachios.
