For the turkey: Two days before cooking, combine the salt with the herbes de Provence, sumac and black pepper in a bowl. Rub all over the turkey, inside and out, and under the skin of the breast and legs where accessible, being careful to not tear the skin. Place on a sheet tray fitted with a rack or in a roasting pan fitted with a rack. Place in the refrigerator, uncovered, for 2 days.
Bring the turkey out of the fridge 1 to 2 hours before cooking to temper.
Preheat the oven to 350 degrees F.
Combine the butter, minced thyme and lemon zest in a bowl and season with salt and pepper. Place the turkey on a work surface. Spread the butter under the skin of the turkey around the legs and breasts, as evenly as possible. Stuff the cavity with the quartered lemons, apple, shallots and sprigs of thyme. Tie the legs closed with kitchen twine. Place the onions, carrots, celery and garlic at the bottom of a roasting pan fitted with a rack. Place the turkey on the rack. Roast, basting every 30 minutes, until an instant-read thermometer inserted in the thickest part of the leg (do not touch bone) reads 155 to 160 degrees F, 12 to 14 minutes per pound, or 2 to 3 hours. Allow the turkey to rest for 90 minutes.
For the gravy: Meanwhile, strain the drippings and juices from the roasting pan into a bowl. Discard the vegetables. Melt the butter in a medium saucepan over medium heat. Add the flour and whisk until the raw flavor is cooked off, about 2 minutes. Whisk in the cider vinegar and soy sauce for depth. Slowly whisk the strained drippings into the flour to avoid clumps. Simmer until the gravy just coats the back of a spoon, 5 to 7 minutes. (The gravy will not be a heavy thick gravy, but rather a light gravy.) Season to taste. Carve the turkey and serve with the gravy.