Prep: 20 minutes; Cook: 15 minutes; Cool: 20 minutes. Beets are delicious raw, but they should be sliced very thinly. The easiest way to do this is to use a Japanese mandoline. If you don't have one, you can also slice them with a vegetable peeler, or try grating beets on a box grater.
How to Make It
Place fillet in a large skillet, and season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Add 1/4 cup rice vinegar and water to reach halfway up fillet. Simmer, covered, 12–15 minutes or until fish flakes easily when tested with a fork. Transfer to a plate, and refrigerate 20 minutes or until cool. Flake fish into bite-size pieces with a fork.
Stir together the sour cream, the horseradish, and remaining 1 tablespoon vinegar, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper in a medium bowl. Add the watercress, and toss to coat.
Divide the watercress evenly among 4 bowls, and top each serving with beets and fish. Sprinkle with chopped dill.