Heat the oil over high heat in a large casserole. Season the lamb and chicken legs with salt and pepper, and brown the lamb meat and chicken on all sides; set the chicken aside.
Add the onions and tomatoes to the lamb, and cook and stir until onions are soft, about 10 minutes. Pour in 3L water and bring to the boil. Reduce the heat and simmer for 1 hour.
Stir in the turnip, celery, swede, paprika, coriander and cumin. Season with salt and pepper. Return the chicken to the casserole, and let simmer for 50 minutes. Add the pumpkin and carrots, and continue cooking over low heat for 20 minutes more.
Dip a small cup of broth out of the casserole, and pour over the sultanas in a small bowl. Stir the courgettes and chickpeas into the casserole and continue to simmer for 20 minutes. If there is not enough broth to cover the vegetables, add a small amount of boiling water to extend it.
When the meat and vegetables are tender, turn off the heat, and leave everything in the casserole to rest, covered. Dip out a small amount of broth with a ladle, and stir the broth into the harissa to use as a sauce. Serve this piquant sauce separately in the same way that you serve the sultanas.
Prepare the couscous according to the packet instructions. In a frying pan or under the grill, quickly cook the Merguez sausages. Place the lamb, chicken, the sausages and the vegetables on a large platter and serve the couscous, harissa sauce and sultanas on the side.