Chop the leeks (watch the video!): Chop off the dark green stems of the leeks, then slice them in half length-wise. Place each leek half cut-side down, then chop it into thin slices, resulting in half-moon shapes. Rinse the slices thoroughly in a colander to remove any dirt.
In a skillet, heat the olive oil or butter over medium high heat. Add the leeks and cook stirring often, about 4 to 5 minutes, until they soften and just before they begin to brown.
Season with a few pinches of kosher salt and fresh ground pepper.