Preparation Whisk together yogurt, horseradish, mustard, and lemon juice. Season with 1/4 teaspoon salt. Cut thin ribbons of cucumber with slicer, then very thinly slice onion. Toss with half of yogurt sauce. Discard skin from fish and break fish into large flakes. Arrange fish on cucumber-onion mixture and serve with remaining sauce.
Preparation Whisk together yogurt, horseradish, mustard, and lemon juice. Season with 1/4 teaspoon salt. Cut thin ribbons of cucumber with slicer, then very thinly slice onion. Toss with half of yogurt sauce. Discard skin from fish and break fish into large flakes. Arrange fish on cucumber-onion mixture and serve with remaining sauce.