Leeks carry a lot of dirt in their layers and if they are not cleaned properly before cooking, you will end up with a very gritty dinner!
1: Trim roots and dark green tops from leek, leaving white and pale green parts remaining
You can see from the second image above that the dirt is embedded through the deeper layers. So you'll need to clean them right through.
Slice leek lengthways (from top to bottom) and clean thoroughly under running water, making sure to rinse out the grit from all of the layers.
Saute leeks in a mixture of butter and olive oil for best flavour