How to Make It
Preheat the oven to 400°. Remove the skin from the trout. On a rimmed baking sheet, rub the skin with olive oil, then bake for about 10 minutes, until crisp. Flake the trout into 1-inch pieces.
In a bowl, whisk the 1/4 cup of oil with the lemon juice; season with salt and pepper.
Using a sharp knife, peel the grapefruits, removing all of the bitter white pith. Working over a bowl, cut in between the membranes to release the sections into the bowl. Cut the sections into 1-inch pieces.
In a medium bowl, gently toss the avocados with 2 tablespoons of the dressing. In a large bowl, toss the arugula, endives and red onion with 2 tablespoons of the dressing. Add the grapefruit pieces, sliced radishes and smoked trout and the remaining dressing and toss gently. Add the avocados. Spread the salad out on plates. Break the trout skin into shards, scatter over the salads and serve.