How to Make It
In a large pot, bring 4 cups of the water to a boil. Add 1 1/4 cups of kosher salt, the sugar, mustard seeds, dried onion, caraway seeds, peppercorns, juniper berries and bay leaves. Stir to dissolve the salt and sugar completely. Remove the pot from the heat.
Line a large stockpot or bucket with 2 very large, sturdy plastic bags. Put the turkey into the bags, neck first. Pour the warm brine over the turkey. Add 1 1/2 cups of the Riesling and 4 quarts of the cold water. Seal the bags; press out as much air as possible. Refrigerate for 2 days.
On Thanksgiving morning, preheat the oven to 350°. Drain the turkey, scraping off the spices, then transfer it to a large roasting pan and let it return to room temperature. Discard the brine.
Add the quartered onion, the garlic and 1 cup of the water to the pan and roast the turkey for 1 1/2 hours. Add the remaining 1 cup of water to the pan and roast for about 1 1/2 hours longer, or until an instant-read thermometer inserted into an inner thigh registers 165°. Cover the breast loosely with foil during the last hour of roasting to prevent it from browning too quickly.
Transfer the turkey to a cutting board. Strain the pan juices into a measuring cup and skim off the fat; reserve 3 tablespoons of the fat. In a bowl, mix the reserved fat with the flour until a paste forms.
Set the roasting pan over 2 burners and heat until sizzling. Add the remaining 1 cup of Riesling and bring to a simmer over moderately high heat, scraping up any browned bits from the bottom and sides of the pan. Strain the wine into a medium saucepan and boil until reduced to 1/4 cup, about 5 minutes. Add the stock and the reserved pan juices and bring to a boil. Whisk in the flour paste and simmer over moderate heat until the gravy thickens slightly and no floury taste remains, about 10 minutes. Season with salt and pepper. Carve the turkey and serve with the Riesling gravy on the side.