Lemony Thyme Butter: Blend lemon zest and garlic in a mini-processor (or a regular food processor) until finely chopped. Add butter, mustard, lemon juice, scallions, thyme, paprika, salt and pepper, and process. The butter can be made two days ahead. Bring to room temperature before using.
Roast Turkey: Generously season the main turkey cavity with salt and pepper, and coat the cavity with 2 tbsp. butter. Starting at the neck end of the turkey, carefully slide your hand between skin and meat of the breast, thighs, and upper drumsticks, to loosen the skin. Spread the Lemony Thyme Butter over the meat of the thighs and drumsticks, below the skin, on both sides. Arrange a few lemon slices under the skin. Spread the remaining butter over the breast meat, below the skin. Fill the main cavity with any remaining lemon slices, parsley, and rosemary. Tie legs together loosely to hold shape. Tuck the wing tips under.
Place the turkey on a rack in a large roasting pan. Rub outside all over with the oil; sprinkle with salt and pepper. Pour stock into the pan. Roast until a thermometer inserted into the thickest part of the thigh registers 165-170 degrees, about 3 hours. Transfer the turkey to a platter. Tent it very loosely with foil, and let it rest for at least 30 minutes (the internal temperature will rise 5 to 10 degrees). Serves 10 to 12