Chefs are scouring cookbooks that are hundreds or even thousands of years old in a bid to re-introduce the foods of yore, which are unprocessed, made with seasonal ingredients, and new to modern eaters. Examples of "historic gastronomy," as some are calling it, include a spiced pigeon from a 1780 recipe served at Dinner by Heston Blumenthal in London; and a 10-course dinner at Pensiero in Evanston, Ill., based on Roman recipes from the 4th and 5th centuries.