Using two pieces from the leek top and some kitchen twine, make a bouquet
garni:  place your favorite herbs (pick mild ones:  the stock should taste
like chicken, not chicken soup; fresh parsley is always a safe choice) on
one piece of the leek top, cover with another piece, and tie together.
Set aside.

Wash and peel the carrot, and wash the leek and celery stick.  Cut into
medium dice.  In a large soup pot, heat the oil over medium heat, then
add the diced vegetables and stir, letting them soften but not brown.
Add the chicken bones and cook for another minute or two.  Pour in white
vermouth and bring to a boil.  When the vermouth starts boiling, add water
to the pot, until the chicken bones and vegies are completely covered.
Return to a boil, then lower the heat, and frequently skim off and discard
the foam that rises to the surface.  After 10 minutes of simmering, add the
bouquet garni and the onion.  Cook for 2 more hours, maybe 2 1/2.  (You
might need to replenish some water as cooking progresses.)

Line a strainer with cheesecloth.  Place over a large bowl and ladle the
stock into the strainer, so that all the vegies and bones are removed and the
stock is saved.  Press the bones with the ladle to extract as much of their
flavor as you can.

That gets you a stock.  For a strong stock, I'd take an ordinary one and
boil it down, until it's reduced to 1/2 its original volume.