Remove the dark green tops. 2. Trim the "hairy" end off of the bottom of the leek, being careful not to remove the entire base Slice the leek in half lengthwise, leaving the base intact. 3. From there, clean the leeks.  My favorite way to do this is to gently spread the layers and hold upright under cold running water.  Be sure to spread each layer well to allow the water to rinse away all the grit.  Blot dry before slicing. 4. Place cut-side down on the cutting board and cut into thinly sliced half moons. Once prepped, use within 1-2 days.