Gather beef bones, beef trimmings, onion, carrot, celery, garlic, bay leaves, peppercorns, and water
Preheat the oven to 425°F
Place beef bones and trimmings on a baking sheet
Roast until deeply browned
Transfer bones and trimmings to a large stockpot
Add chopped onion, carrot, celery, garlic, bay leaves, and peppercorns
Cover with cold water
Bring to a gentle simmer
Skim off foam and impurities from the surface
Reduce heat to low
Simmer for 6 to 12 hours
Add water as needed to keep ingredients covered
Strain the stock through a fine-mesh sieve
Discard solids
Cool the stock quickly
Refrigerate or freeze the stock
Remove solidified fat from the top before using