Mix mustard, fresh ginger and minced garlic in small bowl.
Season cod fillets with salt and pepper. Spread each with 1 teaspoon mustard blend.
Transfer to baking dish; add vegetable broth and white wine. Braise, uncovered, at 425°F for 20 minutes.
Heat butter in skillet. Add onion; cook 4 minutes. Add spinach; cover; cook 3 minutes to wilt. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Serve cod over spinach.