Scrub the mussels really well and remove their stringy beards. Chuck out any mussels that are damaged in any way or that are open and don't close when tapped on the side of the sink.
Heat the oil in a large non-stick frying pan or saute pan - you need something with a lid - over a low heat. Very gently fry the leeks and garlic for 5 minutes until softened but not coloured, stirring occasionally. There's no need to cover the pan. Add a little water to the pan if the leeks start to stick before they are softened.
Pour over the white wine and stir in the creme fraiche and parsley. Add the mussels, cover the pan with a lid and cook for 3-4 minutes or until all the mussels have steamed open, stirring once. Give the pan a good shake now and again. Mix the cornflour and water together to make a thin paste.
Remove the pan from the heat and tip the mussels and their liquor into a colander over a large bowl. Throw away any that haven't opened, then divide the mussels between two warmed wide bowls.
Tip the mussel liquor back into the pan and stir in the cornflour paste. Bring to a simmer and cook for 1 minute, stirring until thickened. Add a bit more salt and pepper to taste and pour the sauce back over the mussels in the bowls.
Serve each bowl of mussels with a thick slice of crusty bread for mopping up the sauce. Heaven!