Whisk together the dry-brine ingredients, using the amount listed for your chosen salt brand. Use only one of the two salt options. Break up any baking-powder clumps. Remove the neck, giblets, packaging, and any leg restraint from the fully thawed turkey. Pat the skin dry.
Spread the mixture evenly over the outside skin of the breast, legs, wings, and back. Do not put seasoning under the skin. Place the whole turkey on a rack over a drip-catching tray and refrigerate uncovered at 40°F or below for 48 hours. If starting with a frozen turkey, thaw it completely in the refrigerator first; those thawing days are separate. The 48-hour brine counts toward the 1–2-day refrigerated holding window after complete thawing or after purchasing a fresh turkey.
On a large, stable cutting board, start at the cavity opening between the breast and one thigh. Cut along that side toward the neck through the ribs with sturdy kitchen shears. Repeat on the other side. These cuts let the breast section hinge open while staying attached near the neck. Fold the breast section away from the back and legs, snipping any remaining ribs or tissue that prevent it from opening. Keep the breast attached at its hinge and leave the backbone in place. Turn skin-side up and press the breast and each thigh flatter, spreading the leg section away from the breast. Tuck the wing tips under the breast, or snip them off at the joint, to keep them from scorching.
Preheat the oven to 425°F and position a rack with room above the turkey. Line a sturdy rimmed sheet pan with foil. Add the carrots, celery, onions, garlic, and fresh herbs, plus optional lemon slices for more flavorful pan drippings. Pour in the chicken broth. Set an oven-safe wire rack over the aromatics and place the turkey skin-side up on it. If desired, brush the skin lightly with the melted bacon fat just before roasting.
Roast at 425°F throughout. Begin checking at 90 minutes and plan on about 105 minutes of roasting. Turkey is safe at 165°F. For more tender dark meat, I prefer taking both leg quarters to 180°F, measured with an instant-read thermometer in the innermost thigh without touching bone. Also check the thickest part of the breast and the innermost wing on both sides; they should each reach at least 165°F. The thermometer determines doneness. If the pan liquid evaporates and the aromatics begin to scorch, add a little broth to the pan without wetting the skin.
Transfer to a carving board and rest for 20 minutes. Separate the leg quarters and wings at their joints. Remove the breast halves from the breastbone and slice across the grain. Separate thighs from drumsticks if desired, then serve.