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 whip.
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.
Garnish with parsley if desired.
MICROWAVE 10 MINUTES on '8', or until heated through.
For ovens without solid state heat control, MICROWAVE 8 MINUTES on HIGH, rotating dish 1/2 turn after 4 minutes.