Soak foie gras in half-milk half-water mixture for two hours. The liquid should cover the foie gras; this will help the foie gras to soften and become workable.
Place figs in a bowl of lukewarm water for 15 minutes; pat dry. Put the figs in between 2 layers of plastic wrap and gently pound flat to about 1/8” (3 mm). Lay the figs on a tray and drizzle with the Vieille Prune d’Alsace.
Open carefully the foie gras and gently remove the nerves and blood vessels.
Place the cleaned foie gras open-faced on a tray. Sprinkle with sea salt (3/4 oz of salt per 2 lb/ 1kg of foie gras), 20 grams (1 oz) sugar and a few turns of freshly ground white pepper from a mill. Drizzle with the Port.
Let marinate one hour in a cool place (not in the refrigerator).
Build the terrine. In a 7 ¾” (20 cm) terrine mold place a ¾” (2 cm) layer of foie gras (use hands to pat evenly). Spoon a few drops of the Vielle Prune d’Alsace from the figs over the foie gras. Press a single 1/8” (2 cm) layer of the figs gently onto the layer of foie gras. Repeat alternating layers until you have 3 layers of foie gras and 2 layers of figs. Press gently and cover with plastic wrap pressed down on top of the terrine and refrigerate for a minimum of 12 hours.
Pre-heat oven to 250° F/ 120°C. Remove the plastic wrap and replace with a piece of foil; cover with the lid of the terrine. Place the terrine in a 1” (2.5 cm) water bath (lukewarm). Place on the lowest rack of the oven for 10 minutes. Turn off the heat and leave in oven for 30 minutes.
Remove the terrine from the oven and the water bath. Remove the lid and foil and remove any excess fat with a spoon and reserve.
Press the terrine gently; cover with plastic wrap and place in the refrigerator for at least 12 hours. Remove the press and wipe the top of the terrine clean. Cover with a thin layer of melted reserved fat. Wrap again in plastic and refrigerate for 3 or 4 days.
To serve: Remove the foie gras from the mold (hold terrine a few seconds in warm water to loosen the sides). Slice ½” (1 ¼ cm) pieces and place on a plate. Sprinkle with Fleur de Sel and fresh white pepper to taste. Serve with a small salad of mixed greens dressed with hazelnut and Dijon mustard vinaigrette accompanied by toasted country bread.