Make Ahead Potatoes

5 pounds potatoes, peeled and quartered
1 (8-oz) container sour cream
1 (8-oz) package cream cheese
1/2 Cup butter or margarine, divided
1/4 cup finely chopped onion or chives
1/4 to 1 TBS salt
1 tsp white pepper
Place potatoes in a large kettle, cover with water and bring to a boil. Reduce heat, cover and cook for 20 to 25 minutes or until potatoes are tender. Drain and place in a large bowl. Add cream cheese, sour cream and 6 TBS of the butter; beat well. Add onion, salt and pepper, stir until mixture is smooth. Spread in a buttered 9 x 13 baking dish. Melt remaining 2 TBS butter and drizzle over the top. Refrigerate or bake immediately covered at 350 degrees for 40 minutes. Uncover and bake 20 minutes longer. If potatoes are made ahead and refrigerated or frozen, let stand at room temperature for 30 minutes before baking.