Bring potato and water to cover to a boil in a large Dutch oven; boil 15 minutes or until tender. Drain and mash. Stir in 1/4 cup butter, cream cheese, and next 4 ingredients until smooth.
Melt remaining 1/4 cup butter in a large skillet over medium heat; add onion and mushrooms, and saute until tender.
Spoon one-third of potato mixture into a lightly greased 2 1/2-quart baking dish; top with halfof mushroom mixture. Repeat layers, ending with potato mixture.
Bake at 350° for 30 minutes or until lightly browned.