Thaw salmon, if frozen. Preheat oven to 400 degrees F. For relish, in a small bowl combine onion, celery, parsley, pickle, pickle juice, the 1 tablespoon oil, the mustard, salt, and sugar. Cover and chill until ready to serve (up to 1 hour).
Rinse salmon; pat dry with paper towels. Measure thickness of salmon. Sprinkle salmon with Cajun Seasoning; rub in with your fingers.
In a large cast-iron or other heavy oven-going skillet heat the 2 tablespoons oil over medium-high heat. Add salmon, skin side up; cook for 2 to 3 minutes or until lightly browned. Turn salmon. Place skillet in oven. Roast until salmon begins to flake when tested with a fork. (Allow 4 to 6 minutes total per 1/2-inch thickness of salmon, including the browning time.)
To serve, stir relish. Serve salmon with relish.
Combine all ingredients in a small bowl. Store in an airtight container.