1. Preheat oven to 350°.
2. Bake potatoes on baking sheet 25-30 minutes or until tender; set aside to cool. Preheat broiler with rack in highest position.
3. Halve cooked potatoes lengthwise. Scoop out flesh, leaving a 1/4-inch border; transfer potato flesh (about 2 cups) to a bowl. Mash potato with sour cream and 2 tablespoons water; season with salt an pepper. Fold in scallions whites and ham; spoon filling into potato shells.
4. Arrange filled potato skins on a baking sheet; sprinkle evenly with cheese. Broil 5 minutes or until cheese is melted. Spinkle with scallion greens before serving, if desired.