01 - In a large mixing bowl, whisk together the melted butter, olive oil, minced garlic, parsley, thyme, soy sauce, balsamic vinegar, black pepper, and salt until fully emulsified and well combined.
02 - Add the cleaned mushrooms to the bowl and toss thoroughly, ensuring each mushroom is evenly coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for a more pronounced flavor.
03 - Preheat an outdoor grill or grill pan over medium-high heat until it reaches proper cooking temperature.
04 - Thread smaller mushrooms onto skewers for easier handling, or arrange larger mushrooms directly on the grill grates in a single layer.
05 - Grill the mushrooms for 8 to 10 minutes, turning occasionally, until they develop golden char marks and become tender. Baste once or twice with the remaining marinade during grilling for extra flavor.
06 - Remove the mushrooms from the grill and transfer to a warm serving platter. Garnish with additional fresh parsley if desired and serve immediately.