Grate 1/4 teaspoon zest from the grapefruit and set aside. Trim the ends from the grapefruit, then cut away the peel and white pith. Working over a bowl, cut along both sides of each membrane with a paring knife to remove the segments. Discard any seeds. Cut the grapefruit segments into pieces. Squeeze the juice from the membranes into the bowl.
Make the dressing: Whisk the sour cream, 2 tablespoons of the grapefruit juice, the grapefruit zest, scallions, 2 tablespoons water and 1/2 teaspoon each salt and pepper in a bowl.
Toss the watercress, endive, lettuce, celery and onion in a large bowl. Add the dressing and toss to coat. Divide the salad among shallow bowls. Top with the reserved grapefruit segments and the smoked trout.
Photograph by Antonis Achilleos
Courtesy of Food Network Magazine