Place butter in 2-cup measure.
MICROWAVE on HIGH, until melted.
Stir onion rings into hot butter until coated.
Place hamburger patties on bottom half of buns.
Top with cheese slices and onion rings.
Arrange on paper towel.
MICROWAVE 30 SECONDS to 1 MINUTE on HIGH, or until cheese starts to melt.
Cover with tops of buns.