Complex sentence: After his priestly ordination, he studied organ with Albert Leblanc, the former organist of Notre Dame Cathedral in Luxembourg, and especially with Augustin Pierson, organist at St. Louis Cathedral in Versailles, whose brother was also a monk at Clervaux.
Split into simpler sentences: After his priestly ordination, he studied organ with Albert Leblanc, the former organist of Notre Dame Cathedral in Luxembourg. He then studied with Augustin Pierson, organist at St. Louis Cathedral in Versailles, whose brother was also a monk at Clervaux.