Preheat the oven to 350°F (175°C).

Place the bones in a roasting pan and roast for 30 minutes, turning once, until browned.

In a large pot, place the browned bones and all the remaining ingredients except the egg whites, sherry, and chopped scallions.

Add salt to taste.

Bring to a boil over high heat and skim off the foam.

Reduce the heat to low and simmer uncovered until reduced by half, about 5 hours, skimming off the foam as necessary.

Pass the broth through a fine-mesh strainer and return it to the pot.

Discard the solids.

In a large bowl, beat the egg whites with an electric mixer until firm but not stiff.

Gradually stir the soup into the egg whites.

Return the mixture to the pot and cook, stirring frequently, over low heat for about 20 minutes.

This process clarifies the soup.

Strain the broth through cheesecloth into a bowl, then return it to the pot.

Add the sherry and season with salt to taste.

Sprinkle with chopped scallions and serve hot.