01 - Set the oven to 425°F.
02 - Arrange beef bones and optional shank in a single layer on a roasting pan and roast for 30-40 minutes, turning once until deeply browned.
03 - Add carrots, celery, onions, and optional leek to the roasting pan and continue roasting for 15 additional minutes.
04 - Transfer roasted bones and vegetables to a large stockpot. Place roasting pan over medium heat, add tomato paste and 1 cup water to deglaze, scraping up browned bits, then pour into stockpot.
05 - Add garlic, bay leaves, peppercorns, thyme, parsley, and salt to the stockpot. Pour remaining cold water over ingredients until submerged.
06 - Slowly bring the mixture to a gentle simmer over medium heat and skim off foam or impurities with a ladle.
07 - Reduce heat to low and simmer uncovered for at least 4 hours, up to 8 hours for intensified flavor, occasionally skimming and adding water to keep ingredients submerged.
08 - Remove from heat and strain stock through a fine-mesh sieve into a large bowl, discarding solids. Cool rapidly using an ice bath if possible.
09 - Chill stock in refrigerator and once cold, skim off any solidified fat on top.
10 - Use immediately or transfer to containers; refrigerate up to 5 days or freeze up to 3 months.