How to Make It
Whisk together crème fraîche, mustard, lemon juice, salt, and several grinds of pepper in a large bowl. Set aside.
Peel root vegetables; cut vegetables into thin strips either by hand using a chef’s knife or in a food processor fitted with the julienne attachment. Toss vegetables in crème fraîche mixture until well coated. Cover and refrigerate 30 minutes or up to 1 day.
Flake fish into bite-size chunks; add fish and parsley to vegetable salad, and gently toss. Add salt and pepper to taste. Transfer salad to a large, shallow serving bowl. Scatter nasturtium over top, and serve.