Prepare the mushrooms by cutting off the stems, then use a spoon to scoop out the brown gills. Use a damp paper towel to clean any dirt off the surface gently.
Use the tip of a paring knife to cut a ½-inch crosshatch pattern on the tops of the mushroom caps, about 1/16-inch deep scores.
In a small bowl, whisk together olive oil, balsamic vinegar, soy sauce, minced garlic, salt, pepper, and garlic powder.
Add mushrooms to a large plastic bag or baking dish. Turn to coat in the marinade and allow to sit for 30 to 60 minutes.
Barbecue Cooking: Preheat grill over medium-high heat 350 to 400ºF (177 to 204ºC). Clean the grill and carefully grease the cooking grates with tongs and a paper towel dipped in oil. Remove the mushrooms from the marinade, then place it on the hot grill, cap-side down. Cover and cook, 4 to 6 minutes. Flip and cook covered for 3 to 5 minutes.Grill Pan Cooking: Cook mushrooms on a grill pan heated over medium-high heat. Grease the grates with oil, then cook for 4 to 5 minutes. Flip and cook until mushrooms are tender, about 2 to 3 minutes.
Notes
Grill Pan cooking: I recommend using garlic powder instead of fresh minced garlic as the pieces will burn in the hot pan. Increase garlic powder to 1/2 teaspoon.
Make it Gluten-Free: Substitute coconut aminos, gluten-free tamari, or liquid aminos instead of soy sauce.