Homemade Beef Stock

0 Roast the bones: Preheat oven to 400°F. Place oxtail and marrow bones on a baking sheet, drizzle with olive oil, and sprinkle lightly with kosher salt. Roast for 45–60 minutes until deeply browned.
1 Deglaze the pan: Remove the bones and transfer to a large stockpot. Pour ~½ cup hot water into the baking sheet, scrape up browned bits (fond), and pour that into the pot.
2 Add aromatics: Add onions, carrots, celery, garlic, peppercorns, and bay leaves to the pot.
3 Add water: Pour in cold water until everything is just covered, leaving a little space at the top.
4 Simmer: Bring to a boil, then immediately lower to a gentle simmer. Skim off any foam in the first 30 minutes.
5 Cook: Simmer uncovered or partially covered for 3–4 hours. Add more water if needed to keep ingredients just submerged.
6 Strain: Remove from heat and strain through a fine-mesh sieve or cheesecloth. Discard solids.
7 Cool & store: Let cool, refrigerate until chilled, then skim off and reserve the fat cap (tallow) if desired.