How to Make It
On a work surface, using a thin sharp knife, cut the trout fillets crosswise with the knife held almost flat against the fish to make very thin slices; leave the intact skin behind. Arrange the slices in a large glass baking dish, slightly overlapping them if necessary.
In a small bowl, combine 1/2 cup of lemon juice with the sugar, juniper berries, whole peppercorns and 2 teaspoons of salt. Pour the marinade over the trout. Cover with plastic wrap and refrigerate for 2 hours.
Using a sharp knife, peel the oranges, limes and grapefruits, removing all of the bitter white pith. Working over a bowl to catch the juice, cut between the membranes to release the sections. Transfer the sections to a large plate and squeeze the membranes over the bowl to release the juice. Discard the membranes.
In a small bowl, whisk the olive oil with the remaining 1 1/2 tablespoons of lemon juice, the mustard and reserved citrus juice. Season with salt and the crushed pepper. Toss the mesclun with some of the dressing and mound on plates. Remove the sliced trout from the marinade, scraping off the seasoning, and arrange on the salads. Garnish with the citrus sections, celery and fennel fronds. Drizzle with the remaining citrus dressing and serve right away.