1. In a small bowl, whisk the buttermilk with the mayonnaise, horseradish, chives and olive oil; season with salt and pepper.
In a large bowl, toss the arugula and apple with two-thirds of the dressing and transfer to a large platter. Add the trout to the bowl and toss with the remaining dressing; season with pepper. Mound the trout on the arugula and serve.
A fruity German Riesling with hints of green apple and vibrant acidity will contrast with the delicate smoky flavors of the trout here.