Cut leek sections lengthwise into matchstick-size pieces.
Heat olive oil in a large non- stick skillet over medium heat; add leek, and cook 5 minutes or until softened. Increase heat to medium-high, and sauté 1 minute or until lightly browned, stirring frequently. Remove from heat, and set aside.
Place chopped potato in a large saucepan and cover with water; bring to a boil. Cover saucepan, reduce heat, and simmer 12 minutes or until tender. Drain, and return potato to pot. Add buttermilk, Neufchâtel, butter, salt, and freshly ground black pepper; mash with a potato masher until smooth.
To serve, place potato mixture in a large serving dish; top with sautéed leeks. Serve immediately.