Preheat oven to 350°F. In a covered large saucepan, cook potatoes for 15 to 20 minutes in enough boiling water to cover until tender; drain. Mash with a potato masher or beat with an electric mixer on low speed. Stir in the pancetta and green onions. Season with the salt and the 1/4 teaspoon pepper. If necessary, add a small amount of milk to reach the desired consistency.
Grease eight 10- to 12-ounce individual casseroles or custard cups or a 2- to 3-quart casserole. Spoon potato mixture into prepared casserole(s). In a chilled medium bowl, beat whipping cream with chilled beaters of an electric mixer on medium speed until soft peaks form; fold in cheese. Spoon cheese mixture over potatoes.
Place casserole(s) on a baking sheet. Bake, uncovered, about 20 minutes or until potatoes are lightly browned and heated through. If desired, sprinkle with additional pepper.
Slice green onions and shred cheese. Place in separate storage containers; cover and chill up to 24 hours.