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 thoroughly combined and emulsified.
02 - Add the cleaned mushrooms to the bowl and toss generously to coat each mushroom completely in the marinade. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more pronounced flavor.
03 - Preheat an outdoor grill or grill pan over medium-high heat until grate marks form quickly on contact.
04 - Thread smaller mushrooms onto skewers for easy 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 golden brown, tender throughout, and marked with distinct grill lines. Baste once or twice with any remaining marinade during cooking.
06 - Transfer the grilled mushrooms to a warm serving platter, garnish with additional fresh parsley if desired, and serve immediately while warm.