Heat a large pot or Dutch oven over medium heat for 2 minutes. Pour in ¼ cup olive oil, and add the rosemary sprig and one of the crumbled chiles. Let them sizzle in the oil about a minute. Turn the heat down to medium-low, and add the sliced onions. Season with ½ teaspoon salt and a pinch of freshly ground black pepper. Cook 2 minutes and stir in the sliced garlic. Continue cooking another 5 to 7 minutes, stirring often with a wooden spoon, until the onion is soft and starting to color.
Add the cavolo nero and 2 more tablespoons olive oil, stirring to coat the greens in the oil and onion. Season with a heaping ¼ teaspoon salt, and cook the greens slowly over low heat for about 30 minutes, stirring often, until they turn a dark, almost black color and get slightly crispy on the edges. Remove the rosemary and chilie before serving.
