Heat oil in a small frying pan and add diced onion. Cook onion until translucent. Remove from heat.
In a large bowl, combine the sour cream, melted butter and can of soup. Stir until the ingredients are mixed well. Add in the remaining ingredients for the casserole including the cooked onions. Transfer the mixture into a greased 3 quar t casserole dish.
Combine the additional crushed cornflake cereal and ¼ cup of butter together in a small bowl. Sprinkle the mixture over top of the casserole.
Bake in the oven uncovered for about 30 minutes, until golden brown on top and heated through. If the top gets too brown, place a piece of foil on top of the dish.
Note: I tend to use two 9x9 dishes when baking this dish. We eat one immediately and freeze the second one since the recipe make so much. If you’re serving a large crowd, go with the 11x17 casserole dish.