In a large pot of boiling salted water cook potatoes until tender and easily pierced with a fork, about 20 minutes. Drain and return to pot.
Preheat oven to 375 degrees. Grease an 8X8 square baking dish or equivalent oval or rectangle dish.
Mash the potatoes using a potato masher or electric hand beater, until they are broken up but still lumpy. Add 1/3 C of the chives/scallions, milk, cream cheese, butter, egg, salt and pepper and mash/beat together until smooth. Stir in 1/2 C cheddar cheese. Taste to see if more salt or pepper is needed. The potatoes should be a little loose/wet as they will dry out while baking, so if not, add a touch more milk.
Spoon potato mixture into the baking dish and smooth the top. Sprinkle with remaining cheese and with remaining chives/scallions. (Make ahead: at this point the mixture can be cooled for 30 minutes, covered and refrigerated for up to 24 hours before baking).
Bake uncovered until starting to brown and a knife in the middle comes out hot about 40 minutes (55 minutes if previously refrigerated). Let rest 10 minutes before serving.
Enjoy!