Preparation Put cavolo nero crosswise into 1/4-inch-wide strips and cook in a large pot of salted boiling water 3 minutes. Reserve 1/4 cup cooking liquid and drain cavolo nero in a colander. Cook onion and 1/2 cup cilantro in oil in a deep 12-inch heavy skillet over moderate heat, stirring, until onion is softened. Add cavolo nero, salt to taste, and reserved cooking liquid and simmer, stirring, until cavolo nero is just tender, 3 to 5 minutes. Stir in remaining 1/2 cup cilantro and season with salt and pepper.