Put the chicken into a bowl, add half a teaspoon of turmeric and mix until all the chicken is covered. Place a large saucepan on the hob on a medium to high heat and add the oil. Fry the chicken in batches, until cooked and brown around the edges. Put back into the bowl and set aside.
Add a little extra oil, about a tablespoon, to the pan. Add the garlic, onion and salt and cook until the onion is soft – this can take 10–15 minutes.
Now add the cumin, chilli flakes and the remaining half a teaspoon of turmeric and cook for a few minutes.
Add the grapefruit juice and zest, along with the browned chicken. Mix through, then add the hot water.
Stir in the cornflour/water mixture, then cover the pan with a lid and cook for 30 minutes.
Meanwhile, put the couscous into a bowl and stir in the salt. Put the butter on top. Pour in the boiling water until it reaches about 1cm above the couscous. Cover, then set aside so that the couscous can soak up the water.
When the chicken stew is cooked, take it off the heat and add the parsley. Serve the stew with the couscous.