His most successful pieces were his fine vocal art songs for solo voice and piano which often used elements of German and Belgian folk music.
78, published 1884),[5][6] a Violin Concerto in D major, Op.87 (1888), a festival march for symphony orchestra, two overtures, and 11 character pieces for piano.
[4] Lassen's operas, Landgraf Ludwig's Brautfahrt (1857), Frauenlob (1861), and Le Captif (1868), did not have lasting success, though his music to Goethe's Faust (1876) gained popularity and was praised by Franz Liszt.
In 1878–79, Liszt combined excerpts from both these works in a single piano transcription, Aus der Musik zu Hebbels Nibelungen und Goethes Faust (S.496).
Lassen's solo songs and duets show a variety of treatment from the folklike Sei nur ruhig, lieber Robin or the songs with dance rhythms to the through-composed Abendlandschaft (with its more interesting modulations and accompaniment) to the rhapsodic and improvisatory Ich hab im Traum geweinet.