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 vigorously to ensure each mushroom is evenly coated. Cover with plastic wrap and refrigerate for a minimum of 30 minutes, or up to 2 hours for a more pronounced flavor.
03 - Set a grill or grill pan over medium-high heat and allow it to reach the proper cooking temperature, approximately 400°F.
04 - Thread smaller mushrooms onto skewers for easier handling, or arrange larger mushrooms directly on the grill grates in a single layer.
05 - Cook 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 for added richness.
06 - Transfer the grilled mushrooms to a warm serving platter, garnish with additional fresh parsley if desired, and serve immediately while warm.