Preheat oven to 425 F. Prick potatoes with fork 4 to 5 times for steam to vent and place directly on middle rack in oven; bake 40 to 60 minutes, until soft when squeezed using an oven mitt. Lower oven temperature to 350 F.
When cooled enough to touch, remove skins from half of the potatoes and quarter the remaining half with a knife. Place potatoes and butter in a large bowl; carefully mash as steam escapes. Add sour cream and milk, mashing to incorporate. Mix in seasoned salt, pepper, white parts of onion, and jack cheese. Transfer to a greased 9x13-inch baking dish, then evenly spread shredded cheddar on top. For soft bacon, spread it on top, too. (For crispy bacon, cook to desired doneness, then spread on top when casserole is done baking.) Tent with aluminum foil and bake for 15 minutes, then remove foil and bake uncovered for another 10 minutes, or until cheese has melted. Remove from oven and top with remaining green onion (and crispy bacon).
Per serving: 375 calories, 19 grams fat (10 grams saturated fat), 39 grams total carbohydrates, 57 milligrams cholesterol, 658 milligrams sodium, 5 grams fiber, 15 grams protein.
