1Cook noodles according to package directions. Meanwhile, place onion in a greased microwave-safe 11-in. x 7-in. dish; dot with butter. Microwave, uncovered, on high for 1 minute; stir. Microwave 1-2 minutes longer, stirring every 30 seconds.
In a bowl, combine soup and milk; stir in tuna and cheeses. Pour over onion. Drain noodles; add to tuna mixture and mix well. Cover and microwave on high for 3-4 minutes or until heated through and cheese is melted.