1. Preheat the grill to medium-high heat.
2. To prepare vinaigrette, combine first 6 ingredients in a large bowl; stir well with a whisk.
3. To prepare salad, drizzle lemon juice over fillets; sprinkle with 2 teaspoons pepper. Place fillets, skin sides up, on a grill rack coated with cooking spray; grill 5 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Remove skin from fillets; discard.
4. Add spinach to vinaigrette in bowl; toss well. Place 2 cups spinach mixture on each of 4 serving plates; arrange 1 fillet and 6 orange slices on top of greens.