• 10 cups (1") cubes crusty country-style bread (1 lb.)
• ½ cup (1 stick) unsalted butter
• 3 medium onions, chopped
• 3 celery ribs, thinly sliced crosswise
• Kosher salt, freshly ground black pepper
• 1½ cups hot Homemade Turkey Stock or store-bought turkey stock or chicken broth
• 4 Tbsp. (½ stick) unsalted butter, softened, plus more for pan and melted butter (if needed) for gravy
• Vegetable oil, for brushing
• 8 cups (or more) warm Homemade Turkey Stock, divided
• 1¾ tsp. Diamond Crystal or 1 tsp. Morton kosher salt, divided
• 1 tsp. freshly ground black pepper, divided
• 6 Tbsp. all-purpose flour
• Special Equipment: A small metal skewer; kitchen string; foil; large flameproof roasting pan with flat or V-shaped rack; bulb baster (optional); instant-read thermometer; 2-qt. glass measuring cup; gravy separator (optional)