Grease a 9x13 inch casserole dish or 3 quart baking dish with butter. Preheat oven to 350 degrees.
Place cubed potatoes in large stock pan. Cover with cold water and salt well. Bring the potatoes to a rapid boil, then reduce the heat to medium. Simmer until the potatoes are fork tender (the chunks should fall apart easily when pieced with a fork.) Drain the potatoes and return to the hot pan. Place over low heat and simmer until all the moisture has evaporated–about 1-2 minutes.
In a large bowl, mash hot potatoes with a potato masher. Mix in cream cheese, butter, sour cream, milk, eggs, onion, salt, and pepper. Beat the potato mixture with a hand held mixer until light and fluffy.
Place potato mixture into prepared casserole dish. Bake for 20-25 minutes, or until warmed through and slightly browned.
For this recipe, you will need 3 pounds of russet potatoes, which is about 12 regular sized potatoes.
To prepare in advance: Assemble the mashed potato casserole as directed. Cover with plastic wrap and refrigerate up to 48 hours. Bake at 350 for 45-50 minutes, uncovered.
Store leftover baked mashed potato casserole in the refrigerator for up to 4 days and reheat individual servings in the microwave, covered with a damp paper towel so the casserole does not dry out.
You can opt to freeze the leftover casserole in an air-tight storage container or freezer bag for up to 1 month. Defrost in the refrigerator. Mix gently to incorporate and then reheat before serving.
Additional Flavor Options:
Cheesy Mashed Potato Casserole: Mix in 1 cup of shredded or cubed cheddar cheese into the potato mixture before spreading in the casserole dish and then top with 1 to 2 cups of additional shredded cheddar cheese and then bake as directed.
Loaded Mashed Potato Casserole: Before baking, top the potatoes with 2 cups shredded cheese and ½ cup crumbled cooked bacon. Bake as directed and then top with minced chives or green onions if desired.
Onion Mashed Potato Casserole: Add in ¼ cup finely minced onions and 1 teaspoon onion powder into the mashed potato mixture before beating. Bake as directed.