Preheat oven to 450°.
To prepare relish, combine first 5 ingredients, tossing well to coat. Arrange fennel mixture in a single layer on a jelly-roll pan coated with cooking spray. Bake at 450° for 30 minutes or until lightly browned, stirring once. Place fennel mixture in a bowl; stir in sugar, capers, and vinegar.
Preheat grill to medium-high heat.
To prepare fish, place striped bass fillets on grill rack coated with cooking spray. Brush fillets evenly with 2 teaspoons olive oil; sprinkle evenly with 1/2 teaspoon salt and 1/2 teaspoon pepper. Grill fish 4 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Keep warm.
To prepare spinach, heat 2 teaspoons oil in a Dutch oven over medium-high heat. Add garlic; cook 2 minutes or until golden, stirring frequently. Add half of spinach, and cook 1 minute, stirring constantly. Add remaining spinach; cook 2 minutes or until wilted, stirring frequently. Sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper.
Place 1/2 cup spinach mixture on each of 8 plates; top each with 1 fillet and about 1/4 cup relish.