- For the vinaigrette: Simmer the cider in a small saucepan over medium heat until it reduces to ½ cup. Pour the reduced cider, vinegar, oil, shallot, honey, and mustard into a glass jar with a tight-fitting lid. Close the jar and shake vigorously to combine. Season with salt and pepper. Shake the jar again just before serving. (Make-ahead note: Store the vinaigrette covered and refrigerated for up to three days. Return to room temperature, shake vigorously, and check the seasoning before serving.)
- For the salad: Toss together the greens, fennel, apple, and onion in a large bowl. Drizzle in enough vinaigrette to moisten and toss to coat. Each leaf should be glossy, but there should be no excess vinaigrette puddled in the bottom of the bowl.
- Arrange the salad on a platter or serving plates. Break the trout into large pieces and arrange over the top. Drizzle with the remaining vinaigrette.