Place the chicken, onions, apricots and sultanas into a slow cooker. In a bowl, whisk together the chicken stock, tomato paste, lemon juice, flour, ginger, cumin, cinnamon, black pepper, curry powder and cayenne. Pour the mixture over the chicken in the cooker. Cover, set the cooker to high, and cook 2 1/2 hours; or set the cooker to low and cook for five hours.
Place the couscous into a saucepan, stir in the boiling water, cover and let stand until the water is absorbed and the couscous is tender, about 5 minutes. Fluff the couscous with a fork. Scoop onto plates, and serve with chicken tagine.