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 one evenly in the marinade. Cover with plastic wrap 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 a consistent 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 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 warm serving platter. Garnish with additional fresh parsley if desired and serve immediately while warm.