In a large pot, melt 4 tablespoons of butter over medium heat. Add finely diced onions, celery, and garlic, sautéing until softened, about 5–7 minutes.

Gently fold in the crab meat and large shrimp, cooking for 3–4 minutes until the shrimp turn pink.

Pour in 4 cups of seafood stock and bring to a gentle simmer. Stir in a bay leaf, thyme, and cayenne pepper; simmer for 10 minutes.

Using an immersion blender, carefully blend the bisque until smooth. Alternatively, blend in batches in a countertop blender.

Return the blended bisque to the pot over low heat, stir in 1 cup of heavy cream, and gently heat for another 5 minutes.

Ladle the bisque into warm bowls and garnish with chopped green onions or parsley if desired.