1. Saute onion in 1 tablespoon butter until translucent.
2. Mix all ingredients, except cornflakes and 4 tablespoons butter, together.
3. Put potato mixture into a 9x13 inch baking pan.
4. Combine cornflakes and butter, and sprinkle evenly over top of casserole.
5. Bake at 350 degrees F for 40-50 minutes or until heated and bubbly.