Ideally, you should look for long, white, unwrinkled stalks, with
bright green tops. Unlike onions and scallions, leeks are pretty much
the same diameter from leaf to root, with no obvious bulb. Try to avoid
leeks that are too large; a 1-inch diameter is thick enough, especially
in the spring when leeks can have a hard fibrous core that must be
removed. The green tops should not be limp or yellowed, and if you have
a choice, eliminate any leeks that are excessively dirty.
The method of keeping leeks white by mounding soil up the stalk tends
to result in dirt seeping down between the leaves, especially during
rainy periods. If they are very dirty, it may have rained just before
they were dug, which can wash even more dirt into the crevices. If
muddy stalks are the only leeks available, don't worry; just take extra
care to clean them well.
It may be difficult in any but a specialty market to find leeks thin
enough to serve whole. If you have a source, choose all the leeks of
the same thickness, about the same as thick asparagus. They are well
worth the expense to try them if available.
Leeks are a cold weather crop and are at their best in the fall and
early winter. I seem to find them in the market all year long, however,
although not in any profusion.
Of serving them whole, count four to five slim leeks per person.