According to Claudia Roden’s A Book of Middle eastern Food, fessih is a fish that has been salted and buried in the hot sand to ripen. When ready, it will be soft and salty. Ms. Roden allows that people in the West may want to use anchovy fillets in lieu of fessih. (Use canned anchovies, or make your own “anchovies”.) Also, the recipe requires some tahini, a paste made of sesame seeds. It can be found canned in shops that handle Greek foods.