Preheat the oven to 180ºc. Put the chicken thighs into a 30cm ovenproof dish or shallow casserole in which they can all lie in a single layer. Add the sumac, ground coriander, half the garlic and all the olive oil and season. Turn everything over with your hands to make sure it's coated, then cover and marinate in the fridge for 1 hour, if you have time. Mix the onions with the chicken and the marinade, then arrange the thighs so they are skin side up. Dot the plums among the chicken.
In a small bowl, stir the honey and pomegranate molasses together with the cayenne pepper, cumin, orange zest and the remaining garlic. Spoon half of this over the chicken and plums. Season and pour the orange juice around.
Roast for 25 minutes, then spoon the rest of the honey and pomegranate molasses over the chicken skin. Return the oven to cook for a final 20 minutes.
For the relish, bash the nuts in a mortar with the garlic and salt. Add the coriander and bash a bit more to break it al down a bit, not grind to a paste, then stir in the olive oil and lemon juice. Season to taste, then soon it over the chicken and scatter with pomegranate seeds.