Step 1: In a medium-sized bowl, whisk together the balsamic vinegar, olive oil, garlic granules, chopped rosemary, salt, and ground black pepper.
Step 2: Place the portobello mushrooms in a large zip-top bag or a shallow dish. Pour the marinade over the mushrooms, ensuring they are well coated. Seal the bag or cover the dish, and let the mushrooms marinate for about 30 minutes.
Step 3: Preheat your grill to medium-high heat.
Step 4: Remove the portobello mushrooms from the marinade, letting any excess drip off. Place the mushrooms gill-side down on the grill. Grill for about 5-7 minutes, then flip and grill for an additional 5 minutes or until they are tender.
Step 5: Remove the grilled mushrooms from the grill and let them rest for a couple of minutes. Slice them, or serve them whole.
Notes
Don’t rush the marinating process; 30 minutes is just the right amount of time to enhance the flavor.