Place butter in 1-quart measure.
MICROWAVE on HIGH until butter melts.
Blend in flour to make a smooth paste.
Gradually stir in milk.
MICROWAVE 2 MINUTES, 30 SECONDS to 3 MINUTES, 30 SECONDS on HIGH, or until thickened, stirring once with wire whisk.
Layer half the macaroni and half the oysters in 1 1/2-quart casserole.
Season with salt and pepper.
Sprinkle with one-third of the cheese.
Pour sauce over and top with remaining cheese.
MICROWAVE 10 MINUTES on '8', or until heated through.
Garnish with parsley, if desired.