1. Preheat oven to 400° F.
2. Lightly Coat a large glass-baking dish with cooking spray. Rinse fish & pat dry with paper towels. Place fish evenly in dish.
3. In a small bowl, mix homemade ranch dip & mustard. Spoon evenly over fish.
4. Bake 20-25 minutes, or until fish flakes with a fork.
5. Meanwhile, place frozen spinach in a microwave-safe dish & microwave to defrost until warm. Drain excess water.
6. Serve baked fish over spinach.