How to Make It
Preheat oven to 425°F. Coat fillets on both sides evenly with ground fennel seeds, 1 teaspoon pepper, and 3/4 teaspoon salt. Heat 1 tablespoon olive oil in a large ovenproof skillet over medium-high. Add fillets; cook until browned on both sides, about 2 minutes per side. Transfer skillet to oven; cook at 425°F until a thermometer inserted in thickest portion registers 130°F to 135°F, 3 to 5 minutes. Transfer fillets to a plate; let rest 5 minutes.
Meanwhile, whisk together vinegar, canola oil, remaining 2 tablespoons olive oil, remaining 1/2 teaspoon pepper, and remaining 1/2 teaspoon salt in a large bowl. Add sliced fennel, apple, onion, and parsley; toss to coat. Divide slaw among 4 plates. Slice fillets; divide among plates. Garnish with fennel fronds, if desired.