Using Canada as your country of residence you have to choose "schedule driven" and than "one way" will appear. Booking by fare driven you can only book return as you pointed out. Using "schedule driven" you can choose "lowest fare", when flights are displayed in the second step.

You can also try aua.com to book the flight. This site is somehow better than LH. Just checked and found a lot of flights in Feb. for less than 100 EUR.

Hope that hepls.
Cheers