Adding Guinness, or any dark beer, to the brine gives the turkey a toasty flavor and helps give the skin a dark brown color.
In a very large pot, combine the mustard seeds, peppercorns and bay leaves and toast over moderate heat until fragrant, about 2 minutes. Add the brown sugar and salt and remove from the heat. Add 4 cups of water and stir until the sugar and salt are dissolved; let cool completely.
Add the onions, bacon, Guinness and 16 cups of cold water to the pot. Add the turkey to the brine, breast side down, and top with a heavy lid to keep it submerged. Cover and refrigerate for 24 hours.
Preheat the oven to 350° and position a rack on the bottom shelf. Lift the turkey from the brine, pick off any peppercorns, mustard seeds and bay leaves and pat dry. Transfer the turkey to a large roasting pan, breast side up. Scatter the onion wedges in the pan and add 1 cup of water. Using toothpicks, secure the bacon slices over the breast. Roast the turkey for about 2 hours, turning the pan occasionally, until an instant-read thermometer inserted deep into the turkey thighs registers 150°. Remove the bacon and return the turkey to the oven. Roast for about 1 hour longer, until the breast is browned and an instant-read thermometer inserted in a thigh registers 170°. Transfer the turkey to a carving board.
Pour the pan juices and onion wedges into a saucepan and boil until reduced to 3 cups, about 5 minutes. Add the turkey stock and return to a boil. In a small bowl, mash the butter to a paste with the flour. Whisk the paste into the gravy and boil until thickened slightly, about 5 minutes.
Meanwhile, cut the bacon crosswise 1/2 inch thick. In a large skillet, fry the bacon over high heat, stirring occasionally, until browned and crisp, about 3 minutes.
Carve the turkey and serve with the gravy and bacon.