Step 1Remove the skin from the duck, setting aside two pieces of breast skin, to be placed on top of the terrine before baking.
Step 2Put the remaining skin in a saucepan with a cover and cook over medium-low heat for 20 to 30 minutes until the fat is rendered from the duck. Cool, then strain the duck fat into a container. Chill the duck fat in the refrigerator until the fat is set. Any unused fat can be kept in the refrigerator for future use.
Step 3Remove the meat from the duck bones. Cut one duck breast into three-fourth-inch cubes and set aside.
Step 4Into a large bowl, grind the remaining duck meat through the small plate of a meat grinder. Stir in the cubed duck meat and minced Serrano ham. Chill the ground meat while preparing remaining ingredients.
Step 5Heat the oven to 350 degrees.
Step 6In a medium skillet, heat 2 tablespoons of chilled duck fat over medium heat, add the onion and saute 3 to 5 minutes, until the onion is tender and begins to brown around the edges. Add the mushrooms and saute 3 to 6 minutes, until the mushrooms begin to brown and are tender and release their juices. Add the garlic during the last minute of cooking. Let cool.
Step 7In a large pot of boiling salted water, blanch the Swiss chard about 1 minute until it turns bright green. Shock in ice water, then drain, pat dry and finely chop.
Step 8Add one-half cup of cold duck fat, the mushroom mixture, chopped Swiss chard, Cognac, thyme, nutmeg, beaten egg, salt and pepper to the ground duck and lightly stir until ingredients are blended.
Step 9Coat a 9-by-3 3/4 -inch terrine with some of the chilled duck fat. Pat the duck mixture into the terrine.
Step 10Put the skin from the duck breast over the top of the terrine. Cover with foil and place the terrine on a baking sheet. Bake it for 30 minutes, then uncover and continue to bake 20 to 30 minutes or until juices from the center of the terrine look clear.
Step 11Let stand at room temperature until cool. Cover and refrigerate overnight.