Homemade beef stock lends rich complexity to our French Onion Soup recipe. Make it the day ahead—you'll have plenty left over to freeze and use in recipes for hearty stews and spicy chilis.
Preheat oven to 425°F. In a roasting pan, combine beef bones, stew meat, carrots, and onion. Drizzle with oil, tossing to evenly coat, and spread in a single layer. Roast, turning everything once halfway through, until meat is browned on all sides, about 45 minutes.
Transfer bones, meat, and vegetables to a stockpot. Place roasting pan over two burners, add 1/2 cup water, and bring to a boil, scraping browned bits from bottom; transfer mixture to stockpot. Add enough water to pot to cover meat and vegetables by about 2 inches (5 to 6 quarts). Bring to a boil, skimming foam and impurities from surface.
Add celery stalks and leaves, peppercorns, thyme, and parsley sprigs. Reduce heat to medium-low and gently simmer, uncovered, until mixture is darkened and reduced slightly, and bones have released their marrow, 8 to 12 hours.
Strain stock through a fine-mesh sieve (line it with cheesecloth for clearest possible stock). Let stand until fat rises to surface, then skim it from stock with a spoon. Freeze fat to use in place of oil or schmaltz in savory dishes. Stock can be refrigerated in an airtight container up to 1 week, or frozen up to 6 months.
To remove the fat without skimming, refrigerate stock until it floats to the top and solidifies, then lift it off with a spoon.