Heat oven to 350°. Soak chicken breasts in buttermilk for about 15 to 30 minutes. Sprinkle each breast with salt and pepper to taste. Roll in crushed cornflakes. Place chicken in a 9x13-inch baking dish. Mix together soup, condensed milk, Worcestershire sauce and onion. Pour mixture over chicken. Cover dish with foil and bake for 40 minutes, uncovering after 25 minutes. Check occasionally to make sure enough liquid remains to keep from sticking. Yields 4to 6 servings.