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.