Brining keeps the turkey moist and juicy during roasting because the brine salt helps the turkey tissue to absorb water and flavorings resulting in a tender bird. You will need a container large enough to submerge the turkey but small enough to fit in the refrigerator. Alternatively, instead of a 20lb bird, use two 10lb birds.
In a large pot, bring four cups of water to a boil then add 1¼ cups of salt, the peppercorns, sugar, caraway seeds, mustard seeds, juniper berries, dried onion and bay leaves. Stir to ensure you completely dissolve the salt and sugar then remove the pot of brine from the heat.
Line a bucket with two clean large garbage bags, one inside the other. Put the turkey into the bags, neck first. Pour the warm brine over the turkey and add 1½ cups of the Chardonnay along with eight pints of cold water. Press out as much air as possible, seal the bags and refrigerate for two days.
Approximately four hours before serving preheat your oven to 350°. Remove the turkey from the brine bags, wipe off all the spices and brine and place it in a large roasting pan where it should be allowed to return to room temperature. Toss out the brine bags and brine.
Add the butter, quartered onion, garlic and one cup of water to the pan and roast the turkey for 1½ hours. Add the remaining cup of water to the pan and roast for about 1½ hours longer, or until an instant-read thermometer inserted into an inner thigh registers 165°. During the last hour of roasting cover the breast with a foil tent to prevent burning.
Transfer the turkey to a cutting board leaving the breast covered with the foil tent. Pour off about half of the pan juices into a bowl and mix with the flour until it forms a wet paste.
Set the roasting pan over 2 burners and heat the other half of the pan juices until sizzling. Add salt and pepper and the remaining one cup of Chardonnay and bring it all to a simmer over moderately high heat. Reduce the Chardonnay to about 1/3 of its original mass all the while scraping up any browned bits from the bottom of the pan. When reduced, stir in the flour paste using a fork. Continue stirring while the sauce simmers over moderate heat until it thickens with no residual flour taste. Carve the turkey and serve with the Amourette Sauce drizzled over the meat.