Beginning April 1, Cathay Pacific will launch a nonstop service several times a week. Not sure if Asiana is responding to that news by having you secure this fare now.
Asiana Airlines have round trip airfares travelling from Seattle (SEA) to Hong Kong (HKG) for $507-$536, making one stop in South Korea. This is the lowest I've seen posted here. See both my samples from Skyscanner [skyscanner.com]
Departs SEA: Sunday, Monday, Tuesday, Wednesday or Thursday beginning Oct 21 thru Dec 4 and then again (same days of the week) from Jan 6 thru May 5, 2019
Returns: wide and good availability
Use the dates and plug it in Skyscanner [skyscanner.com]
to get your choices of the lowest fares with your favorite OTA. See both images. For hotels, check Hotwire's Hot Rates [hotwire.com]