1. Preheat oven to 400°.
2. Remove fronds from fennel bulb; finely chop fronds to measure 1 tablespoon. Remove and discard stalks. Thinly slice fennel bulb. Heat a large nonstick skillet over medium-high heat. Add 1 tablespoon oil; swirl to coat. Add sliced fennel and garlic to pan; sauté 6 minutes or until lightly browned, stirring frequently. Stir in 1/4 teaspoon salt. Remove pan from heat; cool 5 minutes.
3. Score skin of each fish with 3 diagonal cuts. Combine remaining 1 tablespoon oil and lemon juice, stirring with a whisk. Rub inside flesh of each fish with half of lemon juice mixture; drizzle outside of each fish with remaining half of lemon juice mixture. Sprinkle inside flesh evenly with remaining 1/2 teaspoon salt and pepper. Place both fish on a parchment-lined jelly-roll pan coated with cooking spray. Place half of fennel mixture and 4 orange slices inside each fish.
4. Roast at 400° for 30 minutes or until fish flakes easily when tested with a fork; let stand 5 minutes. Sprinkle reserved fennel fronds over fish.