Complex sentence: Verdi was commissioned to write a new opera by the La Fenice opera house in Venice in 1850, at a time when he was already a well-known composer with a degree of freedom in choosing the works he would prefer to set to music.
Split into simpler sentences: Verdi was commissioned to write a new opera by the La Fenice opera house in Venice in 1850. By this time he was already a well-known composer and had a degree of freedom in choosing the works he would prefer to set to music.