Complex sentence: He notably conducted the premiere of Roussel's 4th symphony (which was dedicated to him) with the Orchestre Lamoureux in October 1935; he had previously made the premiere recording of Roussel's 3rd symphony with the Lamoureux.
Split into simpler sentences: He notably conducted the premiere of Roussel's 4th symphony with the Orchestre Lamourex on October 1935, which was dedicated to him. He had previously made the premiere recording of Roussel's 3rd symphony with the Lamoureux.