In a saucepan, cook onion in the 2 tablespoons butter or margarine until onion is tender.
In bowl, combine thawed hash browns, sour cream, condensed soup, onion mixture, and jalapeños, if you like.
Transfer mixture to a 13x9x2-inch baking dish. Sprinkle with the cornflakes and drizzle with the 1/4 cup butter or margarine.
Bake the potatoes in a 350 degree F oven for about 45 minutes or until golden. Season to taste with salt and pepper. Makes 10 to 12 side-dish servings.
Assemble the night before; cover and refrigerate. Then, bake as directed, adding about 10 more minutes to the baking time to heat it through.