Cook potatoes in a saucepan of boiling water for 4 minutes; drain. Arrange one-third of the potatoes in a greased 3-quart casserole dish; sprinkle with 1/2cup cheese, salt and pepper to taste. Repeat layers once; top with remaining potato slices. Set aside. Combine milk and cream in a heavy saucepan; heat just to boiling. Whisk in nutmeg; pour over potatoes. Sprinkle with remaining cheese. Bake at 400 degrees for 30 minutes, or until golden and potatoes are tender. Garnish with green onion.