Combine mustard and dressing, stirring with a whisk.
Arrange fish, skin side down, on a foil-lined baking sheet coated with cooking spray. Brush half of mustard mixture over fish. Broil fish 8 minutes or until fish flakes easily when tested with a fork.
While fish cooks, combine onion and remaining mustard mixture in a large nonstick skillet over medium heat. Cover and cook 2 minutes. Add half of spinach; cover and cook 1 minute or until spinach wilts. Add remaining spinach and tomato; cover and cook 1 minute or until spinach wilts. Stir well to combine. Serve fish over spinach mixture; serve with lemon wedges.