Halve the fennel bulb, remove the tough core, and shave the halves into paper-thin slices using a mandoline or a sharp knife. Place them in ice-cold water for a few minutes; they will curl up and become crisp instead of limp.
Dry-toast the flaked almonds in a skillet until lightly golden, and let them cool on a plate.
Whisk the crème fraîche with the horseradish, the juice of half a lemon, salt, and black pepper. Taste as you go, as horseradish becomes noticeably sharper after sitting in acid for a few minutes than it initially seems.
Thinly slice the radishes. Drain the fennel and pat it dry. Toss the fennel and radishes with the lamb's lettuce, olive oil, and a splash of lemon juice.
Arrange the salad on a plate and flake the smoked trout coarsely throughout. Spoon dollops of the horseradish cream over it, scatter the flaked almonds on top, and finish with plenty of torn dill and black pepper. Smoked trout provides protein and omega-3s, the crème fraîche and almonds supply the fat, and the fennel and radishes fill the plate with fiber-rich vegetables without significant carbohydrates.