Melt 3 tablespoons of the butter in a sauté pan over medium heat. Add the mushrooms in one layer and cook, without moving, until golden brown, about 5 minutes. Sprinkle with salt and pepper and toss. Continue to cook until the mushrooms are cooked through, about 5 more minutes. Add the thyme, garlic and lemon zest, then cook for 1 minute. Add the lemon juice and remaining tablespoon of butter and toss until the butter is melted. Sprinkle with the chopped parsley and serve.