In a large pan, melt the butter over medium heat. Add the onions, celery and bay leaves. Cook until softened, about 3-4 minutes. Add the flour and stir to combine. Continue to cook over medium heat for another 3 minutes, stirring occasionally. Slowly stir in the chicken stock, increase the heat to medium-high and bring to a boil. Reduce heat to medium-low. Simmer , partially covered, for 15 minutes or until the soup is thickened.
Add the sherry, coconut milk, tomato paste, Cajun seasoning and salt. Heat through for 5 minutes. simmer, add the shrimp. Heat through until the shrimp is cooked, about 5-6 minutes. Add the crab the last two minutes of cooking the shrimp, breaking up any lumps.
Ladle into warm bowls and serve with crusty bread.
Notes
Each Cajun seasoning is a bit different, especially when it comes to salt content. The Cajun seasoning that I use does well with 1/2 teaspoon of salt, but yours may not. Add the salt at the end and then adjust according to taste.