Put about 2 inches boiling water in saucepan.
Add onion, celery leaves, bay leaf, 1 1/2 tsp salt, thyme, 1/4 tsp chervil and peppercorns and simmer for 5 minutes.
Drop in washed shrimp.
Add more boiling water if necessary to cover shrimp.
Bring to a boil, turn down heat, cover and simmer just until shrimp are bright red, about 5 minutes.
Remove from heat and drain.
Cool shrimp, shell, clean and chill.
Add gelatin to cold water and let soak 5 minutes.
Combine chicken stock, tarragon and 1/4 tsp chervil in saucepan and heat to boiling.
Remove from heat and add gelatin.
Stir until gelatin is dissolved.
Add vinegar, lemon juice, 1/2 tsp salt, pepper and a tew drops of food coloring to tint the mixture a delicate yellow.
Set in ice water and chill until beginning to set.
Separate the unpeeled avocados into halves and set in a dish or pan, supporting them so they are even.
Spoon a little of the slightly thickened gelatin mixture into the hollows of the avocados and over the cut edges (this will keep the cut part from darkening).
Arrange the chilled shrimp attractively in the hollows of the avocados.
Spoon more gelatin mixture over shrimp.
Chill until serving time.
Garnish each avocado half with a sprig of watercress or parsley at serving time.
Set the salads on individual plates on a bed of watercress if it is available
Note: Use 2 cans of cleaned large shrimp rinsed under cold water and chilled instead of fresh shrimp, if desired.