Preheat oven to 450°F. Roast 5 pounds veal or beef marrow bones (have your butcher saw them into pieces) in a roasting pan, turning occasionally, until browned, 25–30 minutes.
Cut 4 peeled carrots and 4 celery stalks into 3" pieces; add to pan along with 2 halved peeled onions and 1 halved head of garlic. Roast, turning occasionally, until vegetables are brown, 25–30 minutes.
Transfer to a large stockpot; add cold water to cover.
Pour off fat from pan, add 1/2 cup water, and stir, scraping up browned bits; add liquid to pot along with 1/2 bunch flat-leaf parsley stems, 4 thyme sprigs, 2 bay leaves, and 1/4 teaspoons black peppercorns. Bring to a boil, reduce heat, and simmer 4 hours, occasionally skimming foam and fat from surface and adding water as needed. Strain.
DO AHEAD: Stock can be made 2 days ahead.
Let cool; cover and chill, or freeze up to 3 months.
