A 48-year-old woman with major depressive disorder has failed to achieve remission despite adequate trials of sertraline (200 mg for 8 weeks) and venlafaxine XR (225 mg for 10 weeks). Her current PHQ-9 is 16, indicating moderately severe depression. She has no history of mania or psychosis. She is currently on venlafaxine XR 225 mg. The PMHNP is considering augmentation strategies. Which of the following represents the most evidence-based first-line augmentation approach?