Do-Ahead Mashed Potatoes

5 lb. potatoes

2 Tbsp. butter

1 tsp. salt

1/4 tsp. pepper

1 c. sour cream

1 (8 oz.) pkg. cream cheese

1 tsp. onion salt

Boil potatoes and mash with remaining ingredients. Place mixture in greased casserole. Dot with butter and cover. Refrigerate. To serve, bake at 350° fo 15 minutes. Remove cover and bake an additional 15 minutes.