Melt 3 tablespoons of 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 350ºF. 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.