The most important thing to remember about leeks is that they are grown in very sandy
soil. Accordingly, you must rinse them with great care and energy.
Our usual technique:
slice the leek length-wise;
a quick rinse;
cut the leek cross-wise, about 1/3 inch slices;
put the leek slices into a large colander for a much longer (1-2 minutes) rinse
under running water while carefully working the slices through our fingers.
