Oven baked pink ling gratin

Ingredients

• 2 tbsp butter
• 1 cup carrots, finely chopped
• 3 cups leeks, white and light green parts only, cut into rings, divided
• 2 tbsp plain flour
• 1 cup white wine
• 1 cup fish stock
• 1 cup cream
• Salt
• pepper
• 6 x 250g fillets of pink ling
• 1 tbsp tarragon, finely chopped
• 6 tbsp butter
• 2 cloves garlic, minced
• 2 cups breadcrumbs