Melt butter in a large heavy-bottomed saucepan over medium heat. As the butter melts, add olive oil. Just before the butter completely melts, add onion, garlic, and celery. Sauté for about 3-4 minutes until the onion is translucent and the vegetables are soft.
Next, deglaze the pan with dry sherry and continue to cook for about 2 minutes until the liquid has reduced.
Sprinkle flour over the vegetables and whisk until a thick paste is formed. Sauté for about a minute.
Whisk in seafood stock and continue mixing until fully incorporated and no lumps remain. Add bay leaf, Creole seasoning, bouillon powder, cayenne, and white pepper. Mix to fully combine.
Bring the soup to a boil, then reduce the heat to medium-low and simmer until it slightly thickens (10-12 minutes).
Add lemon juice, heavy cream, and half of the crabmeat and continue to simmer, stirring occasionally, until slightly thickened, about 5-8 more minutes. Taste and season with salt and pepper.
Take the pan off the heat and remove the bay leaf. Pour about a tablespoon of dry sherry into each serving bowl, and divide the soup among bowls. Top with remaining crabmeat; sprinkle with fresh parsley or chives, and enjoy.