 His final and 275th performance at the Met was as Kurwenal in Richard Wagner's "Tristan und Isolde" with Jess Thomas and Birgit Nilsson in the title roles.While busy singing at the Met, Cassel also worked as a guest artist with many notable opera companies throughout North America and Europe.He appeared with such American companies as the Pittsburgh Opera, Cincinnati Opera, and the New Orleans Opera, as well as several major companies internationally such as the Vienna State Opera, the Deutsche Oper am Rhein, the Liceu, Ottawa Opera, the Canadian Opera Company, the Palacio de Bellas Artes, the Teatro Massimo, and elsewhere.