Peel potatoes and quarter. Add to stock pot. Cover with water. Bring to a boil and cook for 25 minutes or until fork tender. Drain into a colander.
Put drained potatoes in a large bowl. Add the sour cream, cream cheese, milk butter, salt and pepper. With a hand mixer, beat until almost smooth. Add the egg and beat a bit more until smooth. I add the egg last because I'm afraid of it cooking if added right away to the hot potatoes. Lastly, stir in the cheddar cheese and onions until well combined.
Scoop potatoes into a casserole dish or smaller freezer-baking-safe dishes sprayed with cooking spray (PAM). I sometimes use aluminum loaf pans.
Preheat oven to 375° when you're ready to bake. If you so desire you can add some more cheddar cheese to the top of the potatoes before you bake.Bake for 35-40 minutes or until heated through.
If you are using frozen, be sure to let thaw completely before baking.
Notes
This recipe is one that I typically make ahead and put in the refrigerator 1 - 2 days before I'm going to serve it. Once I put it into a casserole dish I cover it tightly with plastic wrap (I love press-n-seal). Then I take it out of the refrigerator and let it get to room temperature before I bake it.