New England clam chowder

Name: New England clam chowder
Ingredients: 30g unsalted butter; 1 medium onion, finely diced; 2 celery stalks (reserve tender leaves) trimmed, quartered lengthwise, sliced into 1/2cm pieces; 3 tbsps plain flour; 500ml chicken or vegetable stock; 2 (280g) tins chopped clams in juice; 250ml double cream; 2 bay leaves; 450g potatoes, cut into 1.5cm cubes; Salt and freshly ground black pepper; For the pan-toasted croutons; 30-45g unsalted butter; 1/2 baguette, cut into 2.5cm cubes; 3 tbsps freshly chopped flat-leaf parsley