In a large bowl mix together the frozen hash browns, onion, cheddar cheese, sour cream and salt. In a small bowl, stir the chicken and mushroom soups together. Pour the soups over the hash brown mixture and stir together. Transfer to a 9x13 inch baking dish.
Mix the melted margarine and cornflakes together and pour over the top of the casserole.
Bake in a preheated 350 degrees F (175 degrees C) oven for 45 minutes. Bake for an additional 15 minutes if you'd like the casserole more browned.