Place the onions, celery, coriander and parsley in a pan. Add the ginger, cumin, paprika, cayenne pepper, turmeric, salt and pepper. Place the chicken thighs or drumsticks on top; pour in chicken stock and drizzle with olive oil. Simmer, covered, over low heat for 40 minutes.
Preheat oven to 200 C / Gas 6.
Transfer the contents of the pan into an ovenproof tagine or casserole. Add the carrots and sweet potato; cover and cook for 40 minutes, or until vegetables are soft. Add the drained raisins and prunes; cover and cook for another 10 minutes. Serve hot straight from the tagine.