1. In a medium saucepan, melt the butter over moderate heat. When the foam disappears, add the onion. Cook for about 8 minutes, stirring from time to time, until it is tender but not yet browned.

2. Using a wooden spoon, mix the curry powder with the onion. Taste and add more curry if necessary. Cook for 5 minutes, stirring constantly.

3. Remove from heat; incorporate the chicken broth and sherry.

4. Return to heat; bring to a boil, stirring constantly. Remove from heat and reserve.

5. Using a small, sharp knife, cut the avocados in half, lengthwise. Remove the pit. Using a spoon, remove all the pulp; mash it coarsely.

6. Add the avocado pulp, salt, sugar, Worcestershire sauce, and lemon juice to the soup.

7. Blend for 2 to 4 minutes, until the mixture is smooth. Transfer to a medium-sized bowl.

8. Cover the bowl with aluminum foil and refrigerate for at least 1 hour, until the puree is really cold.

9. Just before serving, lightly whip the heavy cream.

10. Using a metal spoon, gently incorporate the cream into the puree, lifting the mass and without turning. Pour the soup into individual bowls; garnish with lemon slices and parsley.