Heat the oven to 200C/180C fan/ gas 6. Halve the nectarines, remove the stone and put all the halves in a roasting tin, cut-side up. Drizzle over the honey and orange juice, tuck in most of the thyme sprigs and bake for 20-25 mins until soft.
Transfer the roasted nectarines to a serving platter or individual bowls. Fold 2-3 tbsp of the roasting juices into the mascarpone, then spoon a little of the mixture into the centre of each nectarine. Drizzle over any remaining liquid, then sprinkle over the pistachios and remaining thyme sprigs. Serve straightaway.
