Back when people went to the Mostly Mozart Festival less for mental stimulation than to escape August heat, they would not encounter “La Passion de Simone” by the Finnish composer Kaija Saariaho. Indeed, the oratorio wasn’t even written when the festival was revamped to cut back on its namesake in favor of more eclectic programming.

Yet it is not a coincidence that “La Passion de Simone” made its premiere in 2006, the 250th anniversary of Mozart’s birth. It was given in Vienna at Peter Sellars’s New Crowned Hope Festival, a festival that — at his insistence — celebrated Mozart by not playing a note of his music. A new work by Ms. Saariaho, currently Mostly Mozart’s composer in residence, was an obvious choice, not just because of her eminence — she has been called the first female composer to enter the top echelons of classical music — but also because of her close relationship with Mr. Sellars, who directed the initial productions of her two operas.

Mr. Sellars even played a role in her becoming an opera composer. Back when she studied electronic music at IRCAM, Paris’s think tank for new music, she didn’t see herself being an opera composer. “It seemed very strange,” Ms. Saariaho said in an interview in Santa Fe, N.M., where her second opera, “Adriana Mater,” had its American premiere at the Santa Fe Opera a few days before. “It still is a big surprise.”

Olivier Messiaen’s opera “St. François d’Assise” was a breakthrough — of sorts. “The first time I saw it, I loved the music, but it seemed an oratorio,” she said. “I saw how people reacted and thought, ‘If Messiaen can’t do it, neither can I.’ But with Peter’s production in Salzburg, it was the same music but a fantastic opera.” Her two operas to date, “L’Amour de Loin,” or “Love From Afar,” and “Adriana Mater,” rank among the most significant recent additions to the art form, although some have faulted their slow, dramatic pace.

When “La Passion de Simone” has its American premiere in the Rose Theater on Wednesday, it will be the first New York performance of one of her large-scale dramatic works. Like the operas, “La Passion” has a libretto by the Lebanese writer Amin Maalouf, but Ms. Saariaho calls it an oratorio. “I had just finished ‘Adriana’ and couldn’t imagine two stage works, one after another, or even how [‘La Passion’] could be one. But Peter liked the idea of doing it onstage,” and it will be seen in his production.

The subject is Simone Weil, the French philosopher and political activist who died in 1943 at age 34. “She was important to me for a long time. Peter knew her work, too.” Might Mr. Sellars have been interested in Weil because of her implications for today’s society and Ms. Saariaho for the intellectual and literary content of her work? “That is an oversimplification,” she said with a smile, “but essentially correct.” “La Passion” is basically a monodrama for solo soprano consisting of 15 episodes that present a “synthesis of Weil’s biography and philosophy.”

Despite the absence of Ms. Saariaho’s dramatic works in New York, her music is no stranger to the city. Her style builds on musical modernism in an elegant and arresting way, while avoiding both avant-garde extremes and overt attempts to curry audience favor. She is not tempted to return to tonality yet emphasizes that her harmonies, like those of tonal music, operate in a functional way. “It is so important for composers to have their own personal harmonic language. It is like a smell — something immediate that is sensed before anything else.”

Ms. Saariaho says she doesn’t much like talking about her music but indicated her willingness do so. Her manner is soft-spoken and her answers always direct. “Some colleagues clarify ideas by speaking about their music, but I don’t talk about it much, even to family.” Critics have seen a cool, Nordic quality to her music — “shimmering, glacial effects” in the words of one — but long ago it took on an expressive content, a quality observable in the lyricism of her operas. Her recent “Terra Memoria” for string quartet, which will be performed August 21, has a strong emotional charge. “Every piece has its history. This one is about how we think of persons who have died and aren’t with us anymore. Its emotional character comes from painful yet beautiful memories.”

She recalls imagining music as a child in Finland. “I didn’t realize that it was anything special. I wanted to be a composer but thought that I didn’t have enough talent. Woman composers hardly existed when I started. At some point, when I was around 20, I decided being a composer is the only thing for me. I wanted desperately to write, but had no technique at all.” Her teacher Paavo Heininen at the Sibelius Academy in Helsinki helped her “discover the tools by analyzing music and doing technical exercises. Composition can be taught, but your personal expression and talent needs to be there.”

Her work at IRCAM helped her understand the physics of sound. “I became more aware of how sounds behave, and this affected my orchestration.” She also tried to reduce her compositional procedures to the computer. “The result was not so spectacular but helped me understand the complexity of a process that cannot be reduced to logical thinking.” She composes at the computer, using the music software program Finale.

She remains based in Paris, where she balances career demands and family. “I am quite good at organizing.” She has two teenage children with whom she speaks Finnish. “I don’t spend a lot of time in Parisian cafés. Composition is very private work. I am the first listener and the music needs to satisfy me. I don’t think about big crowds. I have had so much luck. If I had bad reactions, maybe I would wonder more, ‘How will they react to this one?'”