In a large pot, heat a little oil over medium heat. Add the onion, celery, and garlic, and sauté until softened.

Add the diced potatoes and cook for another 5 minutes, stirring occasionally.

Pour in the fish stock and bring to a boil. Reduce heat and let simmer until potatoes are tender, about 15 minutes.

Add the white fish, shrimp, and mussels. Cook until the seafood is cooked through, about 5-7 minutes.

Stir in the heavy cream and milk, and season with thyme, salt, and pepper. Heat through without boiling.

Serve hot, garnished with fresh parsley.