Cock-a-Leekie, as the British call it, needs the subtle flavor of leeks to make it just right. But in a pinch, onions will work reasonably well.
Combine chicken and giblets with water in soup kettle. Add bouquet garni. Bring to a boil, skim, reduce heat, and simmer, covered, 2 hours. Discard herbs and remove chicken. Cool, skin, and cut meat into bite-sized pieces. Discard skin and bones. Cool stock, chill, and skim fat from top. Return stock and chicken to kettle. Simmer 5 minutes. Add leeks and simmer, partially covered, 20 minutes longer. Season with salt and pepper. Garnish with parsley.