COmbine oyster liquor with 1 cup water in a 2 quart saucepan. Bring to a simmer over medium heat. Add the oyster and simmer until their edges just begin to curl, about 2 minutes. Strain oysters through a fine sieve set over a medium bowl. Reserve oysters and cooking liquid separately.
Heat butter in a 4 quart saucepan over medium-high heat. Add flour and cook, whisking constantly, until goldn brown, 3-4 minutes. Reduce heat to medium, add celery, garlic, onions, parsley, salt, pepper, and cayenne. Cook, stirring frequently with a wooden spoon, until onions and celery are very soft, about 25 minutes.
Stir in milk, cream, and reserved oysters with their cookingliquid and cook, stirring occasionally, until just hot, about 5 minutes. Serve immediately.