Pound stoned ripe olives, anchovy fillets and tuna fish in a mortar with mustard and capers. When the mixture has been blended to a smooth paste, put it through a fine sieve and whisk olive oil into it. Add cognac, and season to taste with freshly ground black pepper.
Cut hard-boiled eggs in half lengthwise and remove yolks. Blend yolks with tapénade mixture, adding a little more