Baked Mushroom-Stuffed Trout

3 tbsp butter
3 tbsp olive oil
6 cups sliced mushrooms
1 1/2 cups finely chopped onion
1 large celery stalk, finely chopped
3 tbsp minced fresh parsley
3 tbsp minced fresh thyme
4 whole trout (about 12 ounces each), cleaned, boned
2 tbsp fresh lemon juice
1/4 cup butter, melted

Melt 3 tbsp 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. Season filling to taste with salt and pepper, set aside to cool.
Preheat oven to 350¡F. Butter large baking sheet. Drizzle lemon juice inside fish, and sprinkle with salt and pepper. Fill cavity with mushroom mixture, dividing equally. Place stuffed fish on prepared sheet. Brush outside of fish with melted butter.
Bake until cooked through, about 30 minutes. Transfer to platter, garnish with lemon wedges.