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.
02 - Add the cleaned mushrooms to the bowl and toss well to coat each one completely in the marinade. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more intense flavor.
03 - Preheat a grill or grill pan over medium-high heat until hot.
04 - Thread smaller mushrooms onto skewers or arrange larger ones directly on the grill grate.
05 - Grill the mushrooms for 8 to 10 minutes, turning occasionally, until golden brown, tender, and beautifully marked with grill lines. Baste once or twice with any remaining marinade during cooking.
06 - Remove the mushrooms from the grill, transfer to a serving platter, and garnish with extra fresh parsley if desired. Serve warm.