Add onions, celery and garlic and continue to cook over low heat, stirring occasionally, until translucent.
Add salt and pepper to taste, pinch of cayenne, thyme and flour, whisking steadily to make a roux. you want this to be light so don't cook to long as
it will get darker.
Slowly whisk in clam juice, whisking until smooth, then sherry, whisking until smooth.
Whisk in cream, whisking until smooth, then whisk in evaporated milk, whisking until smooth.
Raise heat slightly, stir while bringing just until it starts to boil, then lower heat and whisk until slightly thickened, about 5 minutes. Note: you never know how thick a roux makes a dish until it comes to a boil.
Stir in oysters with their liquor, several dashes of Worcestershire sauce and the chopped parsley.
Continue to heat, stirring occasionally, until oysters curl at the edges indicating they are heated through.