Place a large pot or Dutch oven over medium heat.
Add the butter and allow it to melt completely.
Add the minced onion to the melted butter.
Cook for 3 to 5 minutes, stirring occasionally, until the onion is softened.
Add the all-purpose flour to the pot.
Stir the flour into the butter and onion mixture until a smooth roux forms.
Gradually add the seafood stock, stirring continuously to combine it with the roux.
Reduce the heat and bring the mixture to a gentle simmer.
Simmer for approximately 5 minutes, or until the mixture has slightly thickened.
In a small bowl, combine the heavy cream with a few tablespoons of the hot seafood stock mixture.
Stir until smooth. This step tempers the cream and helps prevent it from separating when added to the soup.
Add the tempered cream to the pot.
Increase the heat to medium-low and stir until the cream is fully incorporated and heated through.
Reduce the heat to low.
Add the Worcestershire sauce, lemon zest, mace or nutmeg, Old Bay Seasoning, cayenne pepper, and 1 teaspoon of the salt.
Stir until the seasonings are evenly distributed.
Add half of the lump crab meat to the soup.
Simmer gently for 5 minutes, stirring occasionally.
In a separate small bowl, combine the remaining lump crab meat, the remaining 1/2 teaspoon salt, and the dry sherry.
Stir gently to avoid breaking apart the crab meat.
Ladle the hot soup into serving bowls.
Top each bowl with a portion of the crab and sherry mixture.
Serve immediately while warm.