Cook the leeks in boiling salted water for 8 minutes, or until tender. Drain and place in a shallow heatproof dish. Keep warm.
Melt the butter in a pan, add the onion and fry until soft. Stir in the flour and cook for 1 minute. Gradually blend in the milk. Heat, stirring, until the sauce thickens.
Add the Camembert and heat gently, stirring, until it has melted. Season to taste, then pour the sauce over the leeks. Garnish with parsley and serve immediately.