As it turns out, the simplest of all terrines is also one of the most luxurious and delicious. Foie gras is literally a fattened liver, typically from a goose or duck in its native France, although in this country, raw foie gras is from ducks. Acquiring the fresh, raw foie gras will take more time than preparing it. If you have a high-end gourmet market, you should be able to order a foie gras there. If not, you can buy one online at www.dartagnan.com. Be sure to plan ahead; although there is not much work involved, the terrine requires a total of four days of aging.
Wine recommendations: Rich, silky pâtés call for rich, silky wines. One traditional match is Sauternes or Barsac. Another option is a late-harvest (vendages tardives) wine from Alsace. You don't want something too sweet, but it should have an unctuous quality to stand up to the foie gras.
Set the foie gras on a cutting board or other work surface and let it rest for 30 minutes. Remove it from its package. Separate the large and small lobes, or sections, and remove any visible fat. Make a slit in the bottom of each lobe, open it gently with your fingers, and remove and discard the large vein. If any pieces of the foie gras break off, tuck them into the larger pieces. Season the foie gras all over with salt and pepper. Then press it into a terrine or loaf pan with lid, smooth side up. Press down gently, pour the Cognac over the foie gras, cover with the terrine lid and refrigerate overnight.
Preheat the oven to 200F. Let the foie gras rest at room temperature for 30 minutes. Bring a teakettle of water to a boil. Create a water bath by setting the terrine into a roasting pan, placing it on the middle rack of the oven and filling the pan with enough hot water to come halfway up the sides of the terrine. Bake for 20 minutes. Let the terrine rest, uncovered, for 20 minutes. Pour off any fat that has collected around the edges, and save it for another purpose. Using the lid of the terrine turned upside down, gently press the foie gras down into the terrine. Cover and refrigerate for three days.
To serve, gently loosen the edges of the foie gras with a knife, remove and save the visible fat, slice the foie gras, and serve with croutons or toast triangles.