To spatchcock the turkey: Begin by patting the turkey dry with paper towels. Grab a sharp pair of kitchen shears, then lay the turkey on a flat surface breast-side down with the legs directly in front of you. Use the shears to cut along one side of the rib bone, staying as close to the bone as possible. Repeat on the other side. Remove the backbone (and any giblets inside the bird). You can store these in a freezer safe container and use them to make stock.
Crack the breastbone. Open the turkey up like a book, then flip it over (it should now be breast-side up). Use your hands to press firmly on the center of the breast until you hear a satisfying crack and the bird lies flat.
Tuck & trim. Place the turkey on a wire rack set on top of a foil-lined rimmed baking sheet, tucking the wing tips behind the breasts to prevent burning. If needed, trim excess fat near the tail.
Dry brine overnight (optional, but recommended). Sprinkle 1 Tbsp kosher salt per 4 lb of turkey all over. Refrigerate uncovered at least 8 hours, or up to 2 days in advance. This draws out moisture, then re-absorbs, yielding deeply seasoned crispy skin.
Season the turkey. In a small bowl, combine 1 stick (113g) of softened butter, 2 Tbsp chopped fresh sage, ½ tsp kosher salt, and as much black pepper as you like. Mix thoroughly. Use your hands to separate the turkey breast from the meat (I find it helpful to slide my fingers back and forth from underneath the skin to help loosen it), being careful not to rip the skin. It doesn’t have to be perfect, just go up as far as you can. Place a little sage-butter under the turkey breast skin, then use your hands from on top of the skin to help press it as far up as it’ll go.
Bring to room temperature. Allow the turkey to rest on the counter for at least 1 hour prior to cooking.
For basting: In a medium saucepan, combine 1 stick of butter, ½ cup white wine, 4 sprigs of sage, 6 sprigs of thyme, and 2 smashed garlic cloves. Melt over medium heat, swirling the pan occasionally. Set aside until needed.
Roast the turkey. Preheat an oven to 425°F and set a rack in the middle of the oven. Baste the turkey with the melted butter mixture, then cook for 30 minutes. Reduce the oven temperature to 375°F, baste the bird again, rotate the an 180°, then continue cooking until the internal temperature of the breast registers between 155-160°F, another 40-50 minutes. Baste again 1-2 more times, about 15 minutes apart. Carryover heat will finish the bird as it rests. A larger bird will need more time–expect to add 6-7 minutes per additional pound.
Rest before carving. Allow the turkey to rest for 15-30 minutes before carving, then transfer the meat to a large serving platter.
While the turkey rests, make the gravy: Carefully transfer the turkey drippings to a measuring cup. Heat 3 Tbsp (42g) butter in a saucepan over medium heat. Once melted, make a roux by whisking in 2 Tbsp flour. Whisk constantly for at least 1 minute, until fully absorbed (I strongly prefer a dark gravy to a lighter, more white gravy–feel free to add more flour if you want it thicker). Add the turkey drippings and enough broth to get to 4 cups of liquid. Raise the heat and bring the mixture to a boil. Once boiling, reduce to a simmer and thicken until your desired consistency is reached. Taste and adjust seasoning if needed (it may not need anything if the bird was seasoned well). Serve warm alongside turkey!