Start by blitzing all the marinade ingredients together in a food processor until smooth. In a bowl, combine the chicken thighs and marinade, making sure they are well coated. Refrigerate for a couple of hours or overnight. 

Preheat the oven to 220°C or 200°C fan (Gas Mark 7). Slice the plums into quarters and put half of them in a roasting dish together with the celery sticks, onion and garlic. Top them with the chicken thighs (skin side up) together with their marinade. Season with salt and pepper and roast for 20 minutes. 

Meanwhile, put the quinoa in a large saucepan that comes with a tight-fitting lid and pour over 250ml water. Bring this to a boil over high heat, then turn down and let it simmer gently without the lid for 15 minutes – by this time all water should be absorbed. Turn off the heat, cover the saucepan with a clean tea towel, clamp on the lid and leave to sit for another 5 minutes. 

Remove the dish from the oven and spoon over the residual juices all over the chicken so it does not  dry. Reduce the oven temperature to 200°C or 180°C fan (Gas Mark 6) and return the chicken to the oven for another 10 minutes. Add the remaining plums, sprinkle the brown sugar and return to the oven for a final 10 minutes. 

Fluff the quinoa with a fork and turn into a large serving bowl. Mix with the remaining quinoa salad ingredients and set aside. Remove chicken from the oven and serve with its juices, some fresh tarragon and the Arabian quinoa salad.