1. Combine first 4 ingredients, 1/8 teaspoon salt, and 1/8 teaspoon black pepper in a medium bowl, stirring with a whisk. Remove fronds from fennel bulb; chop fronds to measure 2 tablespoons. Remove and discard stalks. Cut fennel bulb in half lengthwise; discard core. Thinly slice fennel bulb. Add sliced fennel, orange sections, onion, and olives to orange juice mixture; toss gently to coat. Stir in fennel fronds.
2. Heat a large nonstick skillet over medium-high heat. Sprinkle fish evenly with remaining 1/4 teaspoon salt and the remaining 1/4 teaspoon pepper. Add butter to pan; swirl until butter melts. Add fish to pan; cook 4 minutes on each side or until desired degree of doneness. Serve with relish.