To wash leeks: trim off the hairy root end, cut off most of the green tip, cut them in half lengthwise but leave the an inch at the root end in tact to hold it together, rinse under running water or if they are really dirty put leeks in a bowl of warm water and swish around, lift the leeks out of the water.
And always save your leek tops for stock.
The recipe in the link above involves balancing cleaned leeks until soft and then dressing them in a mustard vinaigrette. I like to make sure the dressing gets into the layers of the leeks and marinate for a bit. This is a great dish to make in advance of serving it. Instead of a half of an egg as pictured in the above recipe. I often push the egg through a fine strainer into a bowl and then sprinkle this sieved egg over the marinated leeks when I'm serving them.
This relish is inspired by leeks vinaigrette, but the leeks are incorporated into the dressing. It’s more of a relish, thick and punchy, and is great mixed into hot potatoes, tossed with blanched green beans or asparagus, or on salmon.
Gently sweat a lot of sliced leeks (about 2 cups of cleaned and sliced leeks- the white part) along with a sliced shallot and a few cloves of garlic over low heat in a healthy amount of olive oil until very soft but no color. Season the leek mixture with salt as they cook. They should be crowded in the pan.
Meanwhile, in a medium sized bowl place 1 Tablespoon of wholegrain mustard, 1 Tablespoon of dijon mustard, ground black pepper, the zest of 1 lemon, and a Tablespoon of red or white wine vinegar.
Once the leek mixture has completely collapsed and is very soft and translucent, you can add a splash of white wine if you have an open bottle or want an excuse to open one (but this is optional.) Then add the leek mixture to the bowl of mustards. The olive oil you cooked the leeks in should balance the mustard and vinegar, but taste and adjust and add more olive oil if needed. It should be thick with sweet leeks but punchy from all the mustard and vinegar.