1. Place potatoes in large saucepan and cover with cold water. Boil until tender, about 30 minutes. Drain.
2. In a large mixing bowl, combine all ingredients and beat until smooth. Pour mashed potatoes into greased 9x13-inch casserole dish. (Can be cooked now or cover and refrigerate up to 1 day.)
3. Preheat roaster oven to 400°F. Place dish in roaster oven on insert rack and bake until heated through, about 30 minutes.
For lump-free potatoes, use an electric mixer to beat all ingredients together.
1 1/2 teaspoon of salt and 1/2 teaspoon of onion powder can be substituted for 2 teaspoons of onion salt.