In their latest set of concerts, the Mostly Mozart Festival on Tuesday presented two works: one largely unknown, the other among the most beloved in the standard repertoire. The first, Luciano Berio’s 1989 Rendering, is fragmented and open-ended. The second, Beethoven’s Piano Concerto no. 5, “Emperor”, about as solid and decisive as they come. The MMF Orchestra and conductor Susanna Mälkki were convincing in the first half, and were joined by the extraordinary Garrick Ohlsson for the Beethoven.

Rendering, for those who are unfamiliar with it (as I was), is built around fragments sketched out by Schubert for a symphony in D major, which was to be his tenth. These bits and pieces existed in a two-staff piano score, and Schubert made some indications as to instrumentation. (A strong underpinning in the lower winds and brass at the opening, for instance, seems like artistic license on Berio’s part, but in general his arrangement sounds fairly Schubertian.)

Lovely as they are, the remnants are too few and far between for some simple touching-up to suffice, and questions of authenticity and artistic intent arise as soon as the unfinished work of a great composer is amended by anyone of a later era. Berio’s solution (explained below) quells both practical and artistic concerns. Practically speaking, it provides an opportunity for this lovely music to be performed, without confronting the vagaries of a posthumous “completion”. Artistically, it allows us to enter the minds of both Berio and Schubert, essentially setting the compositional process itself to music.

Berio composed filler material in between the segments of “real” Schubert, a soundtrack to the blank spaces, crossed-out failed attempts, and erasures left unresolved at the end of the Schubert’s brief life. These places in the music are filled with whirring effects, especially in the woodwinds and celesta, and quotations not only from the unfinished work in question, but also from many of Schubert’s other late works. (One of the first such episodes features the primary theme of the first movement of the Piano Sonata in B flat, D.960, played in harmonics by the concertmaster.) It was almost as if we, the audience, were placed within Schubert’s thoughts as he tried to figure out a way forward, juggling possibilities in an effort to make something stick.

The way in which Berio connects this material (what he dubbed the “cement-work”) to the surrounding, coherent sections perfectly illustrates the phenomenon of insight, the “eureka” moment. The meandering inner monologue of the work lulls the listener into a disoriented frame of mind; just at the moment we drift off from the work completely, it launches, magically and inexplicably, into another “finished” strain from Schubert. Many listeners have difficulty relating to contemporary music, but these instants in Rendering set to music a reality of everyday human thought.

The first movement of the “Emperor” Concerto, in a way, is organized by a similar idea, although the different angles in this work are all from Beethoven’s perspective. His themes flow into and out of one another persuasively, and are of course not stylistic opposites (such as Schubert and Berio), but are still radically varied in emotional outlook. The straightforward, martial quality of the forte themes gives way to the softer, pensive secondary material in much the same way that Schubert’s complete musical ideas recede into the jumbled thoughts of Berio; in the heat of battle, Beethoven’s thoughts travel someplace else.

The performance of the Berio served that work well; the purely orchestrated sections of Schubert played with buoyancy and authority in the outer movements – the assertive first-movement Allegro opens with a fanfare rhythm, and the final movement appears from the sketches to be a scherzo-finale hybrid – and tender lyricism in the central Andante. Ms. Mälkki, who serves as Music Director of Ensemble Intercontemporain, led with a clear technique and keen ear for timbres and orchestral balance in both the style of Schubert and the more modern special effects.

The “Emperor” can be repetitious, and it relies heavily on some really banal harmonic and melodic devices (honestly: how interesting is I-V-I-V-I ad infinitum?) Beethoven’s genius, then, was that a work with these tendencies nonetheless became arguably the greatest concerto yet to be written. Those moments in the first movement where perspective changes drastically often impel the soloist to suddenly take a much faster or slower tempo. Mr. Ohlsson, however, did not hew to “tradition” – according to Toscanini, “the last bad performance” – but rather kept the pulse uniform, which in my view drew the contrasts much more sharply. His sound in the second movement, at once luminous and delicate, simply had to be heard to be believed, and a daring, fun concluding Rondo topped off a performance that reminded one why no excessive number of performances can rob such a work of its power.