Cook potatoes in a large pot of boiling, salted water until just tender, about 30 minutes. Drain. Transfer potatoes to bowl. Using an electric mixer, beat potatoes until smooth. Grandually add milk; beat until blended. Season with salt and pepper

Melt butter in heavy large skillet over medium high heat. Add green onions and saute until tender, about 3 minutes. Mix onions into potatoes.

Can be prepared 2 hours ahead. Cover and let stand at room temperature. Before serving, stir over low heat until heated through.