Combine the sherry vinegar, capers and horseradish in a large bowl. Season with salt and pepper. Whisk in the olive oil until emulsified. Add the red onions and toss with the dressing. Add the smoked trout and the mixed greens, and toss until well combined. Season the salad with freshly ground black pepper. Serve immediately.
Recipe courtesy of Gabriele Corcos and Debi Mazar