Method:

• Melt the butter in a large saucepan over a moderate heat. Add the onions and cook gently until softened but not brown.
• Remove from the heat and stir in the flour to make a paste. Slowly add the stock, stirring to break up any lumps.
• Add the celery salt, seasoning, paprika, nutmeg and leeks to pan and stir to mix.
• Return to the heat and bring to the boil, stirring constantly.
• Reduce the heat, cover the pan and simmer for 30 minutes until the leeks are very tender.
• In a small bowl beat the cream and egg yolks together the slowly stir into the soup. Cook gently, stirring constantly for 4-5 minutes until the soup has thickened. Do not allow to boil as the soup will curdle.
• Serve with crutons.