Place the halved onion, cut side down, directly into a dry, ungreased large stockpot over medium-high heat. Cook for 4 to 5 minutes until the cut sides are deeply browned and slightly charred, which will give the broth its classic golden color.

Add the beef roast to the pot with the charred onion and pour in enough cold water to completely submerge the meat, about 10 to 12 cups.

Bring the liquid to a gentle boil over high heat, then immediately lower the heat to medium-low to maintain a soft simmer. Periodically skim off any foam or impurities that rise to the surface with a slotted spoon during the first 15 minutes of cooking.

Stir in the peppercorns, bay leaves, and salt. Partially cover the pot and let the beef simmer very gently for 1.5 hours, making sure the liquid does not boil rapidly so the meat remains tender.

Add the carrots, parsnips, leek, and celery root to the pot. Continue to simmer gently for an additional 30 minutes, or until the vegetables are tender and the beef is easily pierced with a fork.

Remove the beef from the broth and let it rest on a cutting board for 5 minutes before slicing it against the grain into thin pieces. Serve the sliced beef on a warm platter surrounded by the simmered root vegetables, ladle a cup of the hot strained broth over the top, and garnish generously with fresh chives.