his completions of several unfinished mozart works, including the requiem in d minor and "great mass in c minor", are considered his most important achievements. in the mozart requiem, he reconstructed an "amen" fugue from mozart's own sketches. john eliot gardiner commissioned him to write missing orchestral parts to five movements of cantatas by johann sebastian bach, such as "ach! ich sehe, itzt, da ich zur hochzeit gehe". as a performer, he is best known as soloist in classical-era piano concertos in general, and those of mozart and beethoven in particular, in which he robustly re-creates performance practice of the composers' time such as by improvising cadenzas and shorter embellishments in the composers' style.