1) In a medium saucepan, add celery, onions and butter, stirring occasionally. When tender, add garlic and mix together. Stir in flour and single cream. Let simmer for 5 minutes.
2) Stir in fish stock and parsley. Cook for 10 minutes until it thickens. Add prawns or lobster, and let simmer for 10 minutes or until heated through. Stir occasionally. Add salt, to taste.
3) For the homemade croutons, preheat oven to 180C/Gas 4. Cut bread with a heart-shaped biscuit cutter. Place onto a baking sheet and bake until toasted.
When ready, serve soup with croutons.