Roast the beef bones and onion halves in a large soup pot over medium-high heat until they are deeply browned, about 8 to 10 minutes.

Add the carrots, celery, leek, black peppercorns, and thyme to the pot, then pour in 8 cups of cold water to submerge all the ingredients.

Bring the liquid to a gentle boil, then reduce the heat to low and simmer uncovered for 1 hour, occasionally skimming off any foam that rises to the surface.

While the broth simmers, prepare the dumpling batter by beating the softened butter with the egg, salt, and nutmeg in a medium bowl until creamy.

Stir the semolina flour into the butter mixture until fully combined, then let the batter rest for 15 minutes to allow the semolina to absorb the moisture.

Strain the completed beef broth through a fine-mesh sieve lined with cheesecloth into a clean pot, discarding the solids, and season with salt to taste.

Bring the strained broth to a very gentle simmer; do not let it boil vigorously or the dumplings will break apart.

Shape the dumplings using two tablespoons dipped in hot broth, scraping the mixture back and forth to form smooth, three-sided ovals.

Gently slide each semolina dumpling into the simmering broth, cover the pot with a tight-fitting lid, and cook on low heat for 15 minutes.

Turn off the heat and let the dumplings steam in the hot broth for an additional 5 minutes until they are light, fluffy, and doubled in size.

Ladle the clear broth and fluffy dumplings into warm bowls, garnish generously with freshly chopped chives, and serve immediately.