Wash the leeks as explained above, then slice them finely.

Chop the onion.

Place the butter in a pan and melt it over low heat. Add the onion and cook until translucent, stirring occasionally. Do not let it brown; it should remain pale.

Add the leeks and stir well. Cover and let cook for about twenty minutes, stirring occasionally.

Add the cream just before serving. You can set the cooked leeks aside until it’s time to serve, then add the cream.

Let it reduce for about ten minutes, depending on the desired consistency, more or less liquid. The creamed leek thickens slightly as it cools.