Combine first 4 ingredients in a large stockpot; cover with water. Bring to a boil; cover, reduce heat, and simmer 30 minutes. Drain.
Return potato mixture to pan. Add cream cheese, milk, sour cream, salt and pepper; mash with a potato masher. Spoon potato mixture into a 1 1/2-quart casserole coated with cooking spray. Cover and bake at 350° for 45 minutes or until thoroughly heated.
Note: This can be made a day ahead; just cool, cover, and refrigerate. Reheat in oven or microwave.