Westbroek made her operatic debut at the Aldeburgh Festival in 1994 as Mère Marie in Poulenc's Dialogues des Carmélites.
[1] Her roles in Stuttgart included Carlotta (Schreker, Die Gezeichneten), Tosca, Emilia Marty (Janáček, Věc Makropulos), Desdemona (Verdi, Otello), Donna Anna (Mozart, Don Giovanni), Giulietta (Offenbach, Les Contes d'Hoffmann), Marie (Smetana, The Bartered Bride) and The Duchess of Parma (Busoni, Doktor Faust).
[6] Other roles she performed at the ROH include Giorgetta (Il Tabarro), Dido (Les Troyens), Maddalena (Andrea Chénier), Santuzza (Cavalleria Rusticana), Katarina (Lady Macbeth of Mtsensk), Minnie (La Fanciulla del West) Sieglinde (Die Walküre) and Elisabeth (Tannhauser).
[7] Westbroek made her Metropolitan Opera debut on April 22, 2011, singing the role of Sieglinde in the premiere of a new production of Wagner's Die Walküre directed by Robert Lepage.
She became a MET regular, and returned to New York to sing the title role in Francesca di Rimini, Katarina (Lady Macbeth of Mtsensk), Elisabeth (Tannhauser) and Santuzza in (Cavalleria Rusticana).
In the 2018/2019 season she sang Minnie in La Fanciulla del West and Sieglinde in Die Walküre, both of which will be screened in cinemas worldwide.