Preheat oven to 375 degrees. Peel long strips of zest off one orange. Slice half very thinly into julienne. Cut peel and pith away from the orange, following contour of orange. Holding orange over a bowl, cut between membranes, allowing segments to fall into bowl. Squeeze juice into bowl; discard membrane. Repeat with remaining oranges.
Combine walnuts with 1 teaspoon oil, 4 large zest strips, 1/2 teaspoon salt, and a few grinds of pepper on a baking sheet. Roast until golden brown and fragrant, shaking once or twice, 8 to 10 minutes. Increase oven temperature to 425 degrees. Let walnuts cool, then coarsely chop with a few strips of zest.
Combine mustard, 3/4 teaspoon salt, and 2 tablespoons orange juice. Whisk in 3 tablespoons oil. In a second bowl, combine parsley, walnuts, and julienned orange zest. Lift oranges out of juice and add to salad. Drizzle with some of the dressing and toss gently.
Season fish with salt and pepper. Heat a large oven-proof skillet over high heat. Add 1 tablespoon oil, swirling to coat pan. Place skin-side down in skillet and cook 3 minutes. Transfer to oven and cook until opaque, 8 to 10 minutes depending on thickness of fish. Serve fish with salad, drizzled with dressing.