To prepare the leeks, I rinse them well, chop off all of the dark green (usually right where the leaves separate from the stalk) and the bottom 1/2″ of the root end. Now, slice them once from end to end. Lay flat side on the cutting board and cut into 1/2″ wide half-moon shapes. Set in a colander and rinse well under cold water to remove as much mud/sand/grit as possible. Now, fill a very large, deep bowl with cold water and dump in the leeks. Agitate them well with your hands to separate most of the rings from one another. Let these sit about 5 minutes in the cold water so all of the grit settles to the bottom of the water. This is why it’s so important to have a very large, deep bowl. Rinse out your colander really well.
After the leeks have sat and separated from the grit they travelled with, gently skim them from the surface of the deep bowl with a slotted spoon and put them back in the colander. Try not to slosh the water around or you risk scooping up the grit you just worked so hard to remove. Rinse leeks one final time in the colander and let drain.