Parsnip Leek Potato Mash

Wash leeks in a bowl of cold water, agitating water to loosen any sand, then lift leeks out and drain in a colander. Pat dry. Cook leeks in 4 tablespoons butter, covered, in a4- to 5-quart heavy pot over moderate heat, stirring occasionally, until slightly softened, 5 to 7 minutes. While leeks are cooking, peel potatoes and cut into 1-inch pieces. Add potatoes, parsnips, water, salt, white pepper, and nutmeg to leeks and bring to a boil, uncovered, stirring occasionally. Reduce heat and simmer, covered, stirring occasionally, until vegetables are very tender and most of liquid is absorbed, about 30 minutes. Coarsely mash, then serve sprinkled with parsley and topped with remaining tablespoon butter.