How to Make It
In a large enameled cast-iron casserole, heat 2 tablespoons of the olive oil. Season the chicken with salt and pepper and cook over moderately high heat until lightly browned, about 3 minutes per side. Transfer the chicken to a plate.
Add the remaining 3 tablespoons of olive oil to the casserole along with the onion, ginger, cinnamon and 1 teaspoon of freshly ground pepper. Cook over low heat, stirring occasionally, until the onion is softened, about 7 minutes. Arrange the chicken pieces in the casserole in an even layer. Spread the sweet potato slices over the chicken and season with salt and pepper. Sprinkle evenly with the tomatoes, garlic and parsley. Add the water to the casserole and bring to a simmer. Cover and cook the tagine over low heat until the chicken and sweet potatoes are tender, about 50 minutes.
Using a spatula, carefully transfer the sweet potatoes to a large bowl. Add the chicken pieces to the bowl and keep warm. Boil the sauce over high heat until slightly thickened, about 5 minutes. Season with salt and pepper. Pour the sauce over the chicken and sweet potatoes and serve.