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 combined and emulsified.
02 - Add the cleaned mushrooms to the bowl and toss thoroughly to coat each mushroom evenly in the marinade. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more intense 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 grate.
05 - Grill the mushrooms for 8 to 10 minutes, turning occasionally with tongs, until they develop golden grill marks and become tender. Baste once or twice with any remaining marinade during cooking.
06 - Remove the mushrooms from the grill and transfer to a serving platter. Garnish with additional fresh parsley if desired and serve warm.