Two low fares from Portland PDX today are United/Star Alliance to London $555-$569 from December to May 10. Amsterdam is $580-$588 with UA/Star Alliance and Delta/SkyTeam to Amsterdam $571-$588 Dec 10-13 and Jan 1 to April 6.
London is on sale with United/Air Canada for December departures from Dec 8-15 and Dec 28-31, then nearly all dates $555 Jan 8 – May 10.
American and Delta are mid-$700s for PDX to London at time of posting.
United Airlines Portland PDX – San Francisco SFO – London LHR – Vancouver YVR – Portland PDX
Delta Airlines Portland PDX – Amsterdam AMS nonstop flights
Mid-to-late April is peak tulip season for Amsterdam, but if early spring weather is good, there might be tulips planted around the city by the first week of April.