Combine soy sauce, 1 1/2 tablespoons honey, and brown sugar in a heavy-duty, zip-top plastic bag. Add fish; seal bag securely, and marinate in refrigerator 1 hour, turning bag occasionally.
Drain citrus sections, reserving 1/4 cup liquid. Combine citrus with mint; set aside.
Combine reserved 1/4 cup liquid, 1 teaspoon honey, and vegetable oil; set aside.
Remove fish from marinade; discard marinade. Sprinkle fish evenly with salt and pepper.
Coat grill rack with cooking spray; place rack over medium-hot coals (350° to 400°). Place fish on rack; grill, covered, 5 minutes on each side or until fish flakes easily when tested with a fork.
Arrange spinach evenly on 2 serving plates. Place fish on spinach leaves, and spoon citrus mixture around fish; drizzle evenly with reserved dressing.