Gather oysters, butter, onion, celery, flour, milk or cream, salt, black pepper, and optional parsley or Worcestershire sauce
Drain the oysters and reserve their liquor
Melt butter in a pot over medium heat
Add finely chopped onion and celery, then cook until softened
Stir in flour and cook briefly to make a roux
Slowly whisk in the reserved oyster liquor
Add milk or cream and stir until smooth
Heat gently until the mixture is hot but not boiling
Add the oysters and cook just until their edges curl
Season with salt, black pepper, and optional Worcestershire sauce
Serve hot, topped with optional parsley or extra butter