Preparation Wash chopped leek well in a bowl of cold water, then lift out and drain well. Peel potatoes and cut into 2-inch pieces. Cover with cold water in a 3- to 4-quart saucepan. Bring to a boil, then reduce heat and simmer, uncovered, until potatoes are tender, about 18 minutes. Drain and return to saucepan. While potatoes are simmering, cook carrots in a 1- to 1 1/2-quart saucepan of boiling salted water until just tender, 5 to 6 minutes, then drain. Cook leek in butter in a 10-inch skillet over moderately low heat, stirring occasionally, until very tender, about 6 minutes. Add milk, salt, and pepper and simmer, stirring, 2 minutes. Add leek mixture to potatoes and coarsely mash with a potato masher, then stir in carrots.
Preparation Wash chopped leek well in a bowl of cold water, then lift out and drain well. Peel potatoes and cut into 2-inch pieces. Cover with cold water in a 3- to 4-quart saucepan. Bring to a boil, then reduce heat and simmer, uncovered, until potatoes are tender, about 18 minutes. Drain and return to saucepan. While potatoes are simmering, cook carrots in a 1- to 1 1/2-quart saucepan of boiling salted water until just tender, 5 to 6 minutes, then drain. Cook leek in butter in a 10-inch skillet over moderately low heat, stirring occasionally, until very tender, about 6 minutes. Add milk, salt, and pepper and simmer, stirring, 2 minutes. Add leek mixture to potatoes and coarsely mash with a potato masher, then stir in carrots.