1To prepare dressing: Whisk creme fraiche (or sour cream), herbs, horseradish, salt and pepper in a small bowl until combined.
2To prepare salad: Gently toss lettuce, watercress (or arugula) and mache (or mixed greens) in a large bowl. Toss with Herb & Horseradish Dressing to coat. Divide the greens among 4 plates. Flake 1 ounce trout over each salad and sprinkle with scallions and capers (if using).
Make Ahead Tip: Cover and refrigerate the dressing (Step 1) for up to 1 week.
Crème fraîche is a tangy, thick, rich cultured cream commonly used in French cooking. Find it in the dairy section of large supermarkets, usually near other specialty cheeses. Sour cream can be used as a substitute, or you can make your own lower-fat version by combining equal portions of reduced-fat sour cream and nonfat plain yogurt.
Mâche (“mosh”), also known as lamb's lettuce or corn salad, is a tangy green that resembles watercress. Popular in Europe, it is enjoyed in the first salads of spring. Look for it in specialty stores, large supermarkets and farmers' markets.