Place potatoes in large saucepan; add just enough lightly salted water to cover. Bring to a boil. Reduce heat; cover and simmer 15 minutes. Add peas to potatoes; cook 3 to 7 minutes or until potatoes and peas are tender. Drain; toss with oil.
Meanwhile, in small saucepan, melt butter over medium heat. Add leek; cook and stir 5 to 7 minutes or until tender. Add mint, salt and pepper; mix well.
Place potato mixture in serving bowl. Pour leek mixture over vegetables; stir gently to coat.