Combine butter and onion in 1 1/2 quart casserole.
MICRO WAVE 2 to 3 MINUTES on HIGH, or until onion is transparent.
Mix in flour to make a smooth paste.
Stir in milk, oyster stew and hot pepper sauce.
Do not cover.
MICRO WAVE 4 to 6 MINUTES on HIGH, or until thickened.
Carefully mix in ham and noodles.
MICRO WAVE 8 to 10 MINUTES on HIGH, or until heated through.
Arrange tomato slices on top of noodle mixture.
Sprinkle with cheese.
MICROWAVE 1 to 2 MINUTES on HIGH, or until tomato slices are warm.
Let stand 3 minutes, covered.