Charleston She-Crab Soup

1 small onion, minced
4 tablespoons butter
1 pound crabmeat (about 12 crabs)
1 quart milk
2 cups light cream
1/4 teaspoon mace
1/4 teaspoon white pepper
1/2 teaspoon salt, or to taste
1/4 cup dry sherry
2 egg yolks
1/4 cup orange crab roe
Rind of 1 lemon, grated

Saut