At least one day ahead, make the butter. Mix the butter ingredients in a bowl. Refrigerate 4 tablespoons of the butter for the gravy and set the rest aside at room temperature for the turkey.

One day ahead, brine and prepare the turkey. In a plastic container or stockpot large enough to hold the turkey, mix all the brine ingredients (except the turkey) in 3 gallons of cold water, stirring until the salt and sugar are mostly dissolved. Discard the neck and the giblets and trim any excess skin or fat. Trim the tail, if desired. Rinse the turkey and submerge it in the brine for at least 4 hours and no more than 6 hours. If the turkey floats, weight it down with a couple of dinner plates.

Remove the turkey from the brine and pat dry with paper towels. Starting at the top of the breast, run your fingers between the breast and the skin to separate them, being careful not to rip the skin. Once you're halfway down the breast, turn the turkey around and work from the bottom of the breast until you have loosened the skin from the breast, thighs, and as far down the legs as you can reach. Rub the juniper butter under the skin, covering the breast and as much of the legs as possible. Tuck the wings behind the breast and truss the turkey with twine, securing the legs to the body. Set the turkey on a rack in a large roasting pan and refrigerate, uncovered, for at least 6 and up to 24 hours.

Roast the turkey. Position a rack in the bottom of the oven and heat the oven to 350 degrees F. If any brine has dripped from the turkey into the roasting pan, pour it out. Then pour 2 cups of warm water into the bottom of the pan and cover the entire roasting pan with foil. Roast undisturbed for 2 hours; remove the pan from the oven and remove the foil. Roast the uncovered turkey until an instant-read thermometer inserted in the thickest part of both thighs reads 165 degrees Fahrenheit, 45 minutes to 1 hour longer.

Move the turkey to a cutting board, tent with foil to keep warm, and let rest for about 30 minutes.

Make the gravy. Strain the turkey drippings into a fat separator cup (or another clear, heatproof container). Let sit until the fat rises to the top and then separate exactly 2 cups of the turkey juice from the fat - don't use more than that or the gravy will be too salty. Combine the 2 cups juice with the chicken broth and enough water to make 4-1/2 cups liquid.

In a medium saucepan, melt the reserved juniper-ginger butter and the unsalted butter over medium-high heat until foaming. Whisk in the flour and cook, whisking constantly, until the mixture is golden brown, 2 to 3 minutes. Gradually whisk in the liquid, bring just to a boil, and reduce to a simmer. Whisking frequently, continue to cook about 5 minutes longer to meld the flavors. Season to taste with salt and pepper.

Make Ahead: The turkey must be brined and buttered a day ahead. You can make and refrigerate the butter up to 1 week ahead or freeze for 2 months. Bring to room temperature before preparing the turkey.

Because different brands of kosher salt have different densities, be sure to measure by weight. For example, 2-1/2 lb. of Morton brand salt is only about 4-1/2 cups.