Rinse the mussels under cold water, scrubbing any dirt. Remove any beards. In a pot, bring 1 cup of water to a boil, add the mussels, cover, and steam for about 5 minutes until they open. Remove from heat, discard any unopened mussels, and set aside.

Make Soup Base

Sauté Vegetables

In a large pot, melt the butter over medium heat. Add onion, celery, garlic, and carrots. Cook until softened, about 5-7 minutes.

Add Flour and Stock

Stir in the flour and cook for 2 minutes. Gradually whisk in the fish stock or vegetable broth, bringing to a boil.

Add Cream and Season

Reduce heat and stir in the heavy cream, sherry, salt, and pepper. Let simmer for 10 minutes.

Incorporate Mussels

Add the cooked mussels and parsley to the soup. Simmer for an additional 5 minutes.

Prepare Crust

Prep Puff Pastry

Preheat the oven to 400°F (200°C). Roll out the puff pastry and cut it into circles to fit your serving bowls. Brush with the beaten egg.

Bake Pastry

Place the pastry circles on a baking sheet and bake for 15-20 minutes, or until golden brown.

Serve

Assemble Dish

Ladle the mussel bisque into bowls and top with the flaky pastry. Serve immediately.