Slice turkey carcass (bones) into fourths and lay on a baking pan with carrots, celery, and onion. Roast 35 minutes, mixing one time.
Put bones, trimmings, vegetables, 4 quarts of water, peppercorns, thyme, parsley, and bay leaf in a large pot; let mixture boil. Lower heat and simmer 3 hours; while cooking, skim off top layer and dispose of.
Drain stock through a fine chinois into a bowl, removing solids. Chill stock, covered overnight; skim off fat solids.