Melt the butter in a large saucepan over medium-low heat. Add the onion; cook 1 minute. Add the garlic; cook 2 minutes longer. Stir in the chopped greens, tossing to coat with the onion mixture. Add the barley, prosciutto, chicken stock and nutmeg. Heat to boiling. Reduce the heat and simmer, covered, until all the liquid has been absorbed and the barley is tender, 35 to 40 minutes.
If the barley seems too chewy, add more broth and continue to cook. If it is too wet, uncover, raise the heat slightly, and cook until the liquid is absorbed. Then stir in the parsley and chives, and add salt and pepper. Serves 4 to 6.