In medium-size saucepan, place potatoes in enough water to cover and cook, covered, until tender, about 20 to 25 minutes. Drain and slice thin.
Preheat oven to 375°. In electric blender or food processor, combine cottage cheese, egg and pepper by whirling 8 to 10 seconds.
Coat shallow 9-inch casserole or pie pan with cooking spray. Place 1/3 of potatoes in single layer in casserole, spread with 1/2 the cottage cheese mixture, and sprinkle with 1/3 of the scallions and Cheddar cheese. Add another 1/3 of potatoes and spread with remaining cottage cheese mixture. Top with remaining potatoes and sprinkle with remaining scallions and Cheddar cheese. (You can stop here, cover dish, and refrigerate it until you are ready to cook the pie.)
Bake, uncovered, until golden, 30 to 40 minutes.
This tasty main-course pie can be made ahead and baked later.