Clean 5 pounds chicken, rinse if necessary under running cold water, and pat dry with paper towels. Generously season with salt the skin and also the cavity of the chicken.
Cut 7 ounces white toast bread into cubes about ½ inch (1.5 cm) thick. Do the same with 3 ½ ounces cooked ham, cut into small cubes. Rinse 2 handfuls flat-leaf parsley and let it drain. Use a chef's knife to chop the herb roughly.
Prepare 3 eggs as follow: Separate the egg yolks from the whites and put the whites in a clean bowl. Add the egg yolks, 1 ½ teaspoons salt, ¼ teaspoon ground black pepper, and ½ teaspoon shredded nutmeg to ¾ cup whole milk. Using a hand whisk, beat until smooth.
Put the bread cubes in a large bowl. Pour the mixture of milk, egg yolks and spices over them. Stir well and let the liquid soak in. Add parsley and ham. Stir until combined.
Beat the egg whites with a pinch of salt until stiff. The quickest way is to use an electric hand mixer whisk. Gradually add the stiff egg whites to the stuffing, stirring gently by hand each time.
Fill the cavity of the chicken with the prepared stuffing. Close the chicken skin between the legs with wooden toothpicks.
Shape the leftover stuffing into small balls and gently squeeze out any excess liquid. Place in the roasting dish and bake in the oven together with the chicken.
Peel 1 medium onion and cut it into thin wedges. Place them in the bottom of a baking dish, and put pieces of ⅓ stick unsalted butter on top. Pour in about half a cup of water.
Place the stuffed chicken breast side down on a bed of onions and butter. Alongside the chicken, you can place the remaining stuffing, as I mentioneearlier.
Preheat the oven to
350 °F (upper and lower heating). Place the chicken in the oven and roast for one hour. Baste the chicken with the juices as it cooks. If there is not enough liquid in the bottom of the roasting pan, add half a cup of water.
After roasting for an hour, carefully flip the chicken breast side up. If your oven has a fan-forced function, this is the time to turn it on. If not, increase the temperature to 400 °F and continue cooking until the chicken skin turns golden brown. This will take about half an hour more.
Transfer the roasted chicken, including the stuffing that has been cooked around it, to a container, cover it with foil, and let it rest.
Pour the drippings from the baking dish into the pan. Be sure to scrape all the remnants from the bottom and add them to the pan as well.
Heat the pan with the drippings over medium heat. Add ½ Tablespoon all-purpose flour, stirring constantly, until it sticks lightly to the bottom of the pan. This will take about a minute. Then set the pan aside and gradually pour in 1 cup chicken broth (lukewarm). Whisk well to break up any lumps.
Return to the heat, reduce the temperature, and simmer gently for about 20 minutes. The sauce should thicken but still be liquid. If necessary, add a splash of water and stir.
Place a sieve over a clean pot and pour the gravy into the sieve. Using a wooden spoon, stir and gently press the contents into the gravy, which will drip down into the pot. Discard the remaining onions in the sieve. Taste the sauce and, if necessary, season to taste.