Combine oyster liquor and water in a medium saucepan over medium heat.
Allow the liquid to come to a simmer then add the oysters and cook until their edges begin to curl, about 2 minutes.
Strain the oysters over a medium size bowl. Reserve oysters and cooking liquid separately.
Melt the butter in a large saucepan on medium-high heat.
Once the butter begins to foam, add the flour and cook, whisking constantly, until golden brown, about 3–4 minutes.
Reduce heat to medium-low. Then add celery, garlic, onions, parsley, salt, pepper, and cayenne.
Cook, stirring frequently, until onions and celery are very soft, about 20 minutes.
Stir in milk, cream, and reserved oysters with their cooking liquid and cook, stirring occasionally, until just hot, about 5 minutes.
One website says, “when the milk bubbles and edges of the oysters ruffle like the hem of a wave sliding up on a beach, the stew is ready to eat”.
Serve immediately alongside crusty bread.