Fish and spring onion pies

Name: Fish and spring onion pies
Ingredients: 4 medium spring onions (with mature white bulbs); 40 gram butter; 1 medium (120g) carrot, finely chopped; 2 sticks celery, trimmed, finely chopped; 2 tablespoon plain flour; 1 cup (250ml) fish stock; 1/2 cup (125ml) cream; 1 tablespoon lemon juice; 1 teaspoon dijon mustard; 750 gram boneless white fish fillets, chopped; 1 tablespoon chopped fresh dill; 1 tablespoon chopped fresh chives; 1 egg, lightly beaten; 2 sheets frozen butter puff pastry, thawed