Complex sentence: Michael Rosewell began his career at the Vienna State Opera, as assistant to Claudio Abbado before going on to conduct extensively throughout Germany, notably in the opera houses of Kassel, Wiesbaden and the Mannheim, where he held the position of Kapellmeister.
Split into simpler sentences: Michael Rosewell began his career at the Vienna State Opera, as assistant to Claudio Abbado. He then went on to conduct extensively throughout Germany, notably in the opera houses of Kassel, Wiesbaden and the Mannheim, where he held the position of Kapellmeister.