How to Make It
Preheat oven to 400°F. Toss together carrots, celery, onions, garlic, neck, liver, heart, gizzard, oil, salt, and pepper on a large rimmed baking sheet. Roast in preheated oven until well browned, about 1 hour, stirring once halfway through cooking. Transfer mixture to a large stockpot. Add 1/2 cup chicken stock to hot baking sheet, scraping up any browned bits with a wooden spoon; pour mixture into stockpot.
Add ground turkey, 3 cups water, and remaining 21/2 cups stock to stockpot. Bring mixture to a simmer over high; reduce heat to medium, and cook, uncovered, skimming foam occasionally, 45 minutes. Using a slotted spoon, remove and discard as many large pieces as possible. Pour mixture through a fine wire-mesh strainer into a large heatproof bowl; discard solids.
Return strained broth to stockpot. Bring to a boil over high; cook, undisturbed, until reduced to 4 cups, about 20 minutes. Remove from heat; let cool completely, about 2 hours. Cover and refrigerate until ready to use. Skim off rendered turkey fat from chilled broth before using; reserve for Best-Ever Turkey Gravy.