Start by scraping out the gills from each mushroom cap with a spoon, creating space for the marinade and making handling easier. Place the caps on a tray and brush generously with olive oil, ensuring all sides are coated.
In a small bowl, mix the minced garlic, lemon juice, chopped thyme, salt, and black pepper. Whisk together until well combined, then pour half of this marinade over the mushrooms, flipping to coat evenly. Let sit for 15-20 minutes to absorb the flavors.
Meanwhile, preheat your grill to medium-high heat. Once hot, lightly oil the grates with a brush or paper towel dipped in oil to prevent sticking.
Place the marinated mushroom caps on the grill, gill side down first. Cook for about 4-5 minutes until you see distinct grill marks and the caps start to soften.
Flip the mushrooms carefully and continue grilling for another 4-5 minutes. During this time, brush the tops with the remaining marinade and a drizzle of balsamic vinegar for a smoky glaze.
Watch for the mushroom caps to soften, shrink slightly, and develop a deep char with bubbling juices. Once you see those signs, remove the mushrooms from the grill and let them rest for a few minutes.
Serve the grilled Portobello mushrooms warm, garnished with a sprinkle of fresh herbs if desired. Their smoky aroma and tender, juicy texture make for a satisfying main or hearty side dish.
