Henryk Opieński was a Polish musicologist and composer whose piano writing reflects the late Romantic influence of his compatriots. The piece relies on fluid harmonic shifts and a gentle, undulant melodic contour.
Focus on subtle voicing of the melody against the harmonic background to avoid a cluttered sound.
Comparable difficulty and character — good candidates for what to learn next.
Berceuse is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Berceuse lasts about 4 minutes.
Berceuse is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Ave Maria d'Arcadelt by Franz Liszt, Ave Verum Corpus de Mozart by Franz Liszt, Am Rhein by Heinrich Hofmann — see the full similar-pieces list on this page.