Combine oyster with 1 cup water in a saucepan. Bring to a simmer over medium heat. Add the oysters and simmer, about 2 minutes. Strain and reserve oysters and cooking liquid separately. Heat butter in a saucepan over medium-high heat. Add flour and cook, whisking constantly, until golden brown, 3 to 4 minutes. Reduce heat to medium; add mushrooms, garlic, onions, parsley, salt, pepper, and cayenne. Cook, stirring frequently with a wooden spoon, until onions and mushrooms are very soft, about 25 minutes. Stir in milk, cream, oystersaand liquid, stirring occasionally, until just hot, about 5 minutes. Serve immediately. Serves 6