Pat fillets dry with paper towel. Place skin-side down, on foil-lined rimmed baking sheet. Spread 1 tsp (5 mL) mustard on each fillet, season with pepper to taste. Set aside.
In large frying pan, over medium-high heat sauté prosciutto until crisp; remove from pan and set aside.
Add oil to pan; sauté onion and mushrooms over medium-high heat until tender, about 5 minutes. Add spinach; cook for 1 minute or until wilted. Divide among fillets, pressing lightly to keep in place; top each with 1 tbsp (15 mL) each Parmesan and prosciutto. Bake in 475°F (240°C) oven for 10 to 12 minutes or until fish is opaque and flakes easily when tested with fork.