Step 1

Preparation Melt 3 tablespoons butter with oil in heavy large skillet over medium heat. Add mushrooms, onion, celery, parsley and thyme; cook until mushrooms brown and all liquid evaporates, stirring frequently, about 20 minutes. Remove from heat. Stir in prosciutto. Season filling to taste with salt and pepper. Cool. (Filling can be prepared 1 day ahead. Cover and refrigerate.)

Step 2

Preheat oven to 350°F. Butter large baking sheet. Open fish as for book. Drizzle with lemon juice. Sprinkle with salt and pepper. Spoon filling over 1 side of each fish, dividing equally. Fold second side over, enclosing filling. Place stuffed fish on prepared sheet. Brush outside of fish with melted butter.

Step 3

Bake until cooked through, about 30 minutes. Transfer to platter.